1. A white balance adjusting method, comprising steps of: A. obtaining maximum spectral tristimulus values X max , Y max , and Z max in a chromaticity coordinate system for a tested panel, minimum spectral tristimulus values X 0 , Y 0 , and Z 0 in the chromaticity coordinate system of the tested panel, spectral tristimulus values RX q , RY q , RZ q , of respective gray levels of a red color of the tested panel, spectral tristimulus values GX q , GY q , GZ q , of the respective gray levels of a green color of the tested panel, and spectral tristimulus values BX q , BY q , BZ q , of the respective gray levels of a blue color of the tested panel, where q is a gray level number, and X, Y, Z, are the spectral tristimulus values of the tested panel; B. computing the spectral tristimulus values Y 1 to Y max-1 of the middle respective gray levels according to Y max and Y 0 since a brightness exponentially varies with the gray level; C. setting the highest chromaticity of the gray levels, setting polynomial functions f RX (m), f RY (m), f RZ (m), which vary based on the gray levels in the spectral tristimulus values of the red color of the tested panel according to the spectral tristimulus values RX q , RY q , RZ q , of respective gray levels of the red color of the tested panel; setting polynomial functions f GX (l), f GY (l), f GZ (l), which vary based on the gray level numbers in the spectral tristimulus values of the green color of the tested panel according to the spectral tristimulus values GX q , GY q , GZ q , of respective gray levels of the green color of the tested panel; setting polynomial functions f BX (n), f BY (n), f BZ (n), which vary based on the gray level numbers in the spectral tristimulus values of the blue color of the tested panel according to the spectral tristimulus values BX q , BY q , BZ q , of respective gray levels of the blue color of the tested panel, where m, l, n, are the gray level numbers; D. establishing polynomial relationships between the spectral tristimulus values of three colors red, green, and blue in a white field of respective gray levels and the spectral tristimulus values of red, green, and blue of respective gray levels according to the set chromaticity; E. computing a deviation Δm of the gray level values of spectral tristimulus values of the red color in the white field of respective gray levels, a deviation Δl of the gray levels of spectral tristimulus values of the green color in the white field of respective gray levels and a deviation Δn of the gray level values of spectral tristimulus values of the blue color in the white field of respective gray levels to further obtain gray level numbers m′, l′, n′, of three colors RGB (red, green, blue) in the white field of respective gray levels; in the step D, according to the set chromaticity, the established relationships between spectral tristimulus values of three colors RGB in the white field of respective gray levels and the polynomial functions of spectral tristimulus values of red, green, and blue of respective gray levels particularly being: 
 X s =a max *( Y s /b max )= f RX ( m )+ f GX ( l )+ f BX ( n ); 
 Y s =[( s /max) E *( Y max −Y 0 )]+ Y 0 =f RY ( m )+ f GY ( l )+ f BY ( n ); 
 Z s =(1 −a max −b max )*( Y s /b max )= f RZ ( m )+ f GZ ( l )+ f BZ ( n ); where X s , Y s , Z s are spectral tristimulus values, s is a gray level number; the step E including: E1. predetermining the deviation Δm of the gray level values of spectral tristimulus values of red in the white field of respective gray levels, the deviation Δl of the gray levels of spectral tristimulus values of green in the white field of respective gray levels and the deviation Δn of the gray level values of spectral tristimulus values of blue in the white field of respective gray levels; E2. calculating ΔX, ΔY, and ΔZ according to the following formulas: 
 X s ′=f RX ( m+Δm )+ f GX ( l+Δl )+ f BX ( n+Δn ); 
 Y s ′=f RY ( m+Δm )+ f GY ( l+Δl )+ f BY ( n+Δn ); 
 Z s ′=f RZ ( m+Δm )+ f GZ ( l+Δl )+ f BZ ( n+Δn ); 
 Δ X=X s ′−X s ; 
 Δ Y=Y s ′−Y s ; 
 Δ Z=Z s ′−Z s ; if ΔX, ΔY, and ΔZ un-match the above, then go to step E3; if ΔX, ΔY, and ΔZ match the above, then go to step E4; E3. substituting m′=m+Δm, l′=l+Δl, n′=n+Δn, into the following formula:  Δ ⁢ ⁢ m Δ ⁢ ⁢ l Δ ⁢ ⁢ n  =  δ [ f RX ⁡ ( m ′ ) + ⁢ δ [ f RX ⁡ ( m ′ ) + δ [ f RX ⁡ ( m ′ ) + f GX ⁡ ( l ′ ) + f GX ⁡ ( l ′ ) + f GX ⁡ ( l ′ ) + f BX ⁡ ( n ′ ) ] / δ ⁢ ⁢ m ′ ⁢ f BX ⁡ ( n ′ ) ] / δ ⁢ ⁢ l ′ ⁢ f BX ⁡ ( n ′ ) ] / δ ⁢ ⁢ n ′ ⁢ δ [ f RY ⁡ ( m ′ ) + δ [ f RY ⁡ ( m ′ ) + δ [ f RY ⁡ ( m ′ ) + f GY ⁡ ( l ′ ) + f GY ⁡ ( l ′ ) + f GY ⁡ ( l ′ ) + f BY ⁡ ( n ′ ) ] / δ ⁢ ⁢ m ′ f BY ⁡ ( n ′ ) ] / δ ⁢ ⁢ l ′ f BY ⁡ ( n ′ ) ] / δ ⁢ ⁢ n ′ δ [ f RZ ⁡ ( m ′ ) + δ [ f RZ ⁡ ( m ′ ) + δ [ f RZ ⁡ ( m ′ ) + f GZ ⁡ ( l ′ ) + f GZ ⁡ ( l ′ ) + f GZ ⁡ ( l ′ ) + f BZ ⁡ ( n ′ ) ] / δ ⁢ ⁢ m ′ f BZ ⁡ ( n ′ ) ] / δ ⁢ ⁢ l ′ f BZ ⁡ ( n ′ ) ] / δ ⁢ ⁢ n ′  - 1 *  Δ ⁢ ⁢ X Δ ⁢ ⁢ Y Δ ⁢ ⁢ Z  to obtain new Δm, Δl, Δn, and go to step E2; E4. outputting the gray level numbers s, m+Δm, l+Δl, and n+Δn.