Abstract: One of the important goals for transforming a color image to a gray one is to try to save as more visual characteristics of the original image as possible.The nonlinear global mapping method utilizes the trigonometric series approximation method, and achieves good results in many cases.By introducing t and t² to mix with the trigonometric series where t is the corresponding parameter, this paper presents an improved nonlinear global mapping method.When the parameters of tand t² are zero, the new method is degenerated into the traditional one.The new method achieves the final result by simultaneity optimizing the parameters of trigonometric series, t and t².Experiments show the obvious improvement of the new method, i.e., even in the case that some feature is blurred or lost by using the traditional one, the new method can also achieve a fairly good result.