Objective To study color space conversion model and methods from L*a*b* to CMYK. Methods In this paper, on the basis of the theory of the plane equation, according to the thought that K equaled to a given value, by adopting the method of fitting in Matlab, the correlation coefficients of four color plane equations were obtained, finally the mathematical model from L*a*b* to CMYK color space was established. Results The color space conversion model from L*a*b* to CMYK was realized, and it could output images based on the model, at the same time, the color difference range was verified to be reasonable. Conclusion The structure of the model was relatively simple and the speed of computing was fast. This paper discussed the model and methods from L*a*b* to CMYK ,which could provide reference for the study of color space conversion.