1. A method, used in an ultrasonic color imaging system, for eliminating abnormal blood flow velocities in a color flow image, comprising the steps of: A. receiving signals of blood flow which are obtained by auto-correlation processing and include strength and velocity of blood flow; B. pre-processing the received signals to generate velocity signs in real time, each of the velocity signs corresponding to each pixel of the color flow image and indicating direction of blood flow at the corresponding pixel; C. obtaining a plurality of the velocity signs in a predefined spatial and temporal order, which includes the velocity sign corresponding to a pixel to be confirmed, and performing weighted smoothing on the plurality of the velocity signs; D. determining the direction of blood flow at the pixel to be confirmed according to result of the weighted smoothing.