In tricolor grating color photography,the digital frequency f_s is a key parameter,which affects the decoding speed and the quality of the decoded image. Therefore,the value of fs should be determined. The triangle relation among digital frequency f_s,gratings frequency f_g and gratings sampling frequency f_gs is analyzed in accordance with the Nyquist sampling theorem. The method to determne the optimum value of parameter f_s is obtained. The result shows that f_s should be 2032 d/i when fg is 20 line/mm. It has not only kept the original encoded image information completely but also improved the decoding speed to 3 times. It is of great benefit to the development of tricolor grating digital color photography and its further application in the future.