Hydraulic focusing technology is an important method in many researches about particles. But there is no reported direct real-time detection method to survey its stability. A novel and high resolution method is put forward, which is based on digital image processing technology to detect hydraulic focusing stability. And five parameters are designed to assess its stable performance. A set of experiments is implemented with different sample and buffer hydraulic focusing forming velocities, and the results and experimental error are discussed seriously. The conclusion can be drawn by the final experimental error analysis which is less than +/- 3% during all the detection course. It can be proved that this hydraulic focusing stability detection method based on digital image process technology has the advantages of the excellent precision, the fast processing speed and the good reliability.