The invention relates to a judging method for determining cavitation inception by adopting a computer program, first provides a method for judging a cavitation state by the variation trend of a time domain middle peak value of an underwater sound signal, i.e. the difference of a maximum value and a minimum value of the signal, along with cavitation coefficient and also provides an algorithm for eliminating an interference signal containing large errors in real time. The variation trend of a signal peak value along with the cavitation coefficient, which is suitable for all types of water turbines and used for judging the cavitation inception, is firstly discovered through repeated experimental research on different types of model water turbines; and the generation position of the cavitation inception of runner blades of the model water turbines can be very accurately determined by utilizing the characteristic of generating local maximum values in the peripheral area of a reflected underwater sound signal peak value positioned at a position generating the cavitation inception.