This article proposed a liquid phase method to synthesize the small, even sized nano lead sulfide sol in high power (2KW) pulse ultrasonic reactor, which is designed by our research group. The characteristics of this method are the short time, the small granularity, the equal particle size and the mild synthesis condition. The particle size of lead sulfide sol is about 7～8 nanometer and the size distribution is narrow. Ultraviolet and visible spectroscopy shows that the absorption limit of this lead sulfide sol is about 670nm. Compared with the normal sized lead sulfide, the absorption limit is greatly blue shifted because of the quantum size effect.