Pseudo-color coding of gray image is a typical processing in the fields of medicine, engineering and military. This paper proposed a new method of pseudo-color equi-density coding of gray image based on tri-primary colors of RGB and pixel's self-transformations. In the method, the negative Pixel f' (x,y) and the positive-negative superimposed pixel f'' (x,y) is easily obtained from the original(positive) pixel f(x,y) in an arbitrary space (x,y). Then the three signals for RGB which drive the physical display device are specified fo f(x,y), f' (x,y) and f'' (x,y) individually, and are fed separately into the red, green and blue guns of the RGB color cube of the monitor. The theoretical analysis and experimental results is also given. This method has advantages of being easy to realize and high color sensitivity for gray levels.