The invention, which belongs to the technical field of automatic measurement, provides a hybrid intelligence soft measuring method of overflow granularity index in a wet grinding process. A hybrid intelligence soft measuring device for overflow granularity index in the wet grinding process includes a ball grinder, a hydraulic cyclone, a pump well, an underflow pump, a valve, a flow meter, a densimeter, a pressure meter, a belt of the grinder for feeding, a water pipe line for feeding, a feed pipe of the hydraulic cyclone, an overflow pipe, a data acquisition unit and a computer. The hybrid intelligence soft measuring method includes the steps of: (1) choosing of auxiliary variables; (2) obtaining of sampled data; (3) soft measuring of grinding granularity index based on case inference; (4) soft measuring of the grinding granularity index based on neural net; (5) solving of certainty factor based on case inference; (6) final solving of the grinding granularity index based on expert rule inference. By the method in the invention, the grinding granularity index can be estimated according to real-time data in a normal grinding process. The granularity index measuring method is the granularity measuring means having the advantages of small relative error, high credibility, high practical value and low cost.