In this paper, the properties of the gray level mixing function in two-dimensionalchaotic map image encryption are analyzed. We also discuss the necessity of introducingparameters and the methods how to present diffusion mechanism and interfuse a pseudo-randomnumber sequence in the gray level mixing function. We proposed a new kind of gray level mixingfunction which not only have diffusion mechanism but also have random variable. It is shown inexperiment that this method has good performance.