In view of the fact that the cavitation generated in the operation of the turbine affects its energy conversion, with the centrifugal pump reverse used as a turbine, the numerical simulation method is used to simulate the internal flow law of the turbine under five different speeds and different cavitation allowance, thus obtaining the cavitation characteristic curves and internal flow field characteristics of the turbine under different speeds. Based on the analysis of the cavitation law at different rotational speeds, the optimal performance threshold of the turbine energy conversion is obtained, followed by the establishment of the critical cavitation coefficient relationship between rotational speed and turbine cavitation. The results show that the rotational speed is positively correlated with the critical cavitation coefficient of the turbine; the turbine works above the critical cavitation coefficient; the cavitation bubbles gather at the tail pipe with the turbine severely cavitated, which is not conducive to the outflow of fluid and results in a reduction of the energy conversion.