Abstract: The cavltation erosion during the operation of jet pump can reduce pump efficlency and the operating life of pump, So no cavitation erosion is an important condition that the jet pump is chosen during application. In order to distinguish and predict whether cavitation erosion will occur or not during the operation, the calculating expression of ultimate flow ratio Mc which can distinguish cavitation erosion characteristics under normal conditions was derived from the Bernoulli equation, and furthermore, the calculating expression of ultimate submersion rate fh which can guarantee against occurrence of cativation erosion during the operation was also obtained. The ultimate constant Ic of cavitation erosion was discussed by comparing the calculated result with Brown method. It was known that the theoretical value of Ic should be 1. 0 and the effect of density ratio on cavitation characteristics was analyzed.