Abstract: Automatic threshold choice is difficult and important to color images edge detection, First of all, on the base of human visual properties, threshold versus intensity function of human visual system are researched and analyzed, by using method of mathematic modeling and the result of saturation research, function of least color difference that can be perceptible by visual systems in detecting edge is given. Finally, we use the principle of wavelet edge detection and suggest an algorithm of automatic threshold choice according to difference color background in wavelet edge detection. Emulate experiments demonstrate good adaptability and edge recognition effeteness of the algorithm.