1. A CIE Lab color space based color conversion method, characterized by comprising: inputting original diagram data based on CIE Lab three-dimensional color space; equally dividing the space in which all the colors corresponding to the original diagram data are located into n two-dimensional color planes of the color space according to lightness so as to form n two-dimensional color planes L 1 , L 2 , L 3 ,. .. , Ln- 1 , and Ln, where n is a nature number; determining specific most saturated points An, Bn, Cn, Dn,. .. of the original diagram data on an outer perimeter of the two-dimensional color plane Ln and determining specific most saturated points An′, Bn′, Cn′, Dn′,. .. on an outer perimeter of the target two-dimensional color plane Ln′ that is of the same lightness level as the two-dimensional color plane Ln of the original diagram data, wherein the specific most saturated points An′, Bn′, Cn′, Dn′,. .. on the outer perimeter of the two-dimensional color plane Ln′ and the specific most saturated points An, Bn, Cn, Dn,. .. on the outer perimeter of the two-dimensional color plane Ln form a first one-to-one corresponding relationship; determining a first conversion matrix according to the first one-to-one corresponding relationship between the specific, most-saturated outer perimeter points An′, Bn′, Cn′, Dn′,. .. of the two-dimensional color plane Ln′ and the specific, most-saturated outer perimeter points An, Bn, Cn, Dn,. .. of the two-dimensional color plane Ln and converting the two-dimensional color (a, b) of one of the specific, most-saturated outer perimeter points of the two-dimensional color plane Ln into two-dimensional (a′, b′) of the point of the target two-dimensional color plane Ln′ corresponding to said one specific, most-saturated outer perimeter point according to the first conversion matrix; determining a second one-to-one corresponding relationship between specific most-saturated points An- 1 ′, Bn- 1 ′, Cn- 1 ′, Dn- 1 ′,. .. on outer perimeter of a target two-dimensional color plane Ln- 1 ′ and specific most-saturated points of the original diagram data An- 1 , Bn- 1 , Cn- 1 , Dn- 1 ,. .. on outer perimeter of the two-dimensional color plane Ln- 1 according to the first one-to-one corresponding relationship, determining a second conversion matrix according to the second one-to-one corresponding relationship, and converting the two-dimensional color (a, b) of one of the specific, most-saturated outer perimeter points of the two-dimensional color plane Ln- 1 into the two-dimensional color (a′, b′) of the point of the target two-dimensional color plane Ln- 1 ′ that corresponds to said one specific, most-saturated outer perimeter point according to the second conversion matrix; calculating the two-dimensional color (a, b) of specific, most-saturated outer perimeter points Ax, Bx, Cx, Dx,. .. of a two-dimensional color plane Lx having a lightness level between the two-dimensional color plane Ln and the two-dimensional color plane Ln- 1 according to the two-dimensional color (a, b) of the specific, most-saturated outer perimeter points An, Bn, Cn, Dn,. .. of the two-dimensional color plane Ln and the two-dimensional color (a, b) of the specific, most-saturated outer perimeter points An- 1 , Bn- 1 , Cn- 1 , Dn- 1 ,. .. of the two-dimensional color plane Ln- 1 , wherein Ax=(a Ax , b Ax ), Bx=(a Bx , b Bx ), Cx=(a Cx , b Cx ), DX=(a Dx , b Dx ),. .. and calculating the two-dimensional color (a′, b′) of specific, most-saturated outer perimeter points Ax′, Bx′, Cx′, Dx′,. .. of a two-dimensional color plane Lx′ having a lightness level between the two-dimensional color plane Ln′ and the two-dimensional color plane Ln- 1 ′ according to the two-dimensional color (a′, b′) of the specific, most-saturated outer perimeter points An′, Bn′, Cn′, Dn′,. .. of the two-dimensional color plane Ln′ and the two-dimensional color (a′, b′) of the specific, most-saturated outer perimeter points An- 1 ′, Bn- 1 ′, Cn- 1 ′, Dn- 1 ′,. .. of the two-dimensional color plane Ln- 1 ′, wherein Ax′=(a Ax′ ′, b Ax′ ′), Bx′=(a Bx′ ′, b Bx′ ′), Cx′=(a Cx′ ′, b Cx′ ′), Dx′=(a Dx′ ′, b Dx′ ′),. .. ; determining a third conversion matrix between the two-dimensional color (a*, b*) of the two-dimensional color plane Lx and the target two-dimensional color (a*′, b*′) of the two-dimensional color plane Lx′ according to the specific, most-saturated outer perimeter points Ax′, Bx′, Cx′, Dx′,. .. of the two-dimensional color plane Lx′ and the specific, most-saturated outer perimeter points Ax, Bx, Cx, Dx,. .. of the two-dimensional color plane Lx; applying the third conversion matrix to calculate a color-converted and adjusted target color for the color of one point of the original diagram data on the two-dimensional color plane Lx; and outputting or storing target diagram data corresponding to color-converted and adjusted target color.