Polished, grinded, and milled samples made of 40Cr stainless steel were prepared for the cavitation erosion experiment. A typical phenomenon of “pits chain”, which consisted of two contact pits and a smaller pit on the ridge between them, was found on the sample surfaces after 15-minute experiment. Numerical simulation indicated that the pressure fluctuation caused by the sequentially formed pits on the solid surface was the main reason for the formation of the “pits chain”. It proves that the early-formed pits affect the formation of the subsequent erosion pits, and the whole cavitation erosion process is not a probability event. Based on the numerical analyses, the development of erosion is divided into four stages, which describe how a pit develops vertically and horizontally under the effect of the pressure perturbation. The development was validated by the characteristics of the damaged surface observed at different experimental time.