1. A method for displaying a multi-view three-dimensional (3D) image, adapted to be applied to a flat-panel display screen and a view separation device composed of parallax barriers for 3D image displaying, comprising: performing a multi-view combination process upon a plurality of single-view images V Λ so as to form and display a multi-view combined 3D image Σ n on the flat-panel display screen; and generating a parallax barrier that is composed of a plurality of transparent elements and a plurality of shield elements, and is used for enabling a view separation effect upon the multi-view combined 3D image Σ n for enabling 3D effect; wherein, the multi-view 3D image combination process is established using the following formulas: Σ n = ∑ i = 0 M - 1 ⁢ ∑ j = 0 N - 1 ⁢ V Λ ⁡ ( i , j ) ( 1 ) and ⁢ ⁢ Λ = Mod [ int ( j - Π × int ⁡ ( i Q ) - Δ m ) , n ] ( 2 ) Λ = Mod [ int ( ( N - 1 ) - j - Π × int ⁡ ( i Q ) - Δ m ) , n ] ( 3 ) and the parallax barrier is established using the following formulas: tan ⁢ ⁢ θ = q ⁢ D H D V ( 4 ) D H = mP H ( 5 ) D V = QP V ( 6 ) B H = D H ⁢ L H D H + L H = Z 0 - L B Z 0 ⁢ D H ( 7 ) B _ H = ( n - 1 ) ⁢ B H ( 8 ) L H = D H ⁢ B H D H - B H = Z 0 L B ⁢ B H ( 9 ) Z 0 = D H D H - B H ⁢ L B ( 10 ) B V = B H tan ⁢ ⁢ θ ( 11 ) L V = L H tan ⁢ ⁢ θ ( 12 ) S = D V × tan ⁢ ⁢ θ = qD H ( 13 ) and, the flat-panel display screen is composed of N×M sub-pixels, in which N represents the total number of sub-pixels in a horizontal direction (X axis) of the display screen, and M represents the total number of sub-pixels in a vertical direction (Y axis) of the display screen; in addition, the horizontal position and the vertical position of any single sub-pixel in N×M display screen are represented respectively using the index i and j, whereas 0≦j≦N−1 and 0≦i≦M−1 and each single sub-pixel has a size of P H ×P V , whereas P H represents the horizontal width of a single sub-pixel and P V represents the vertical height of a single sub-pixel; and moreover, V Λ (i, j) represents the sub-pixel image data of a single-view image V Λ at position (i, j) of the display screen; A represents a view number, and 0≦Λ<n, while n is the total amount of view; m is a positive integer representing a number of sub-pixels of a smallest display unit in horizontal direction, while Q is a positive integer representing a number of sub-pixels of a smallest display unit in vertical direction; Δ is an integer representing a horizontal displacement phase; Π is an integer representing a horizontal displacement amplitude; and int is a round down integer function, and Mod is a function of taking a remainder; and B H represents a horizontal width for each transparent element, and B H represents a horizontal width for each shield element; θ is a slant angle for the strip parallax barrier; Z 0 is an optimum viewing distance; L B is a distance to the parallax barrier; L H is the optimum horizontal interval between two neighboring optimum viewing points; L V is the optimum vertical interval between two neighboring optimum viewing points; D H is the width of a smallest display unit in horizontal direction; D V is the width of a smallest display unit in vertical direction; q is a real number representing a rate of inclination; and S is an increasing displacement of the parallax barrier; wherein, the flat-panel display screen and parallax barrier are determined to increase an optimal dimensional relationship for displaying a multi-view combined 3D image.