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, from a parameter estimation module of the ultrasonic color imaging system, signals of blood flow which are obtained by auto-correlation processing and include strength and velocity of blood flow, wherein the velocity includes a speed and a direction of blood flow; B. pre-processing the received signals, using pre-processing circuitry of the ultrasonic color imaging system, to generate processed velocity signs in real time, each of the processed velocity signs corresponding to a respective pixel of the color flow image and indicating direction of blood flow at the corresponding pixel, wherein pre-processing the received signals comprises selectively changing indications of the direction of blood flow at each pixel based on a determination of reliability of the received signals and a comparison of velocity signs of adjacent pixels along a scan line, as either the direction of blood flow for the corresponding pixel indicated by the velocity included in the received signals or an opposite direction of blood flow for the corresponding pixel indicated by the velocity included in the received signals; C. obtaining, at spatial-temporal smoothing circuitry of the ultrasonic color imaging system, a plurality of the processed velocity signs in a predefined spatial area and temporal order corresponding to multiple time frames of the color flow image, which includes the processed velocity signs from the multiple time frames corresponding to a pixel to be confirmed, and performing weighted smoothing only on the plurality of the processed velocity signs without smoothing velocity magnitude values; and D. determining, using the spatial-temporal smoothing circuitry of the ultrasonic color imaging system, the direction of blood flow at the pixel to be confirmed according to a result of the weighted smoothing; and E. displaying a color on a display device at a location corresponding to the pixel to be confirmed, wherein the displayed color is based on the determined direction of blood flow at the pixel to be confirmed.