Abstract: Fuel cladding is the first barrier to confine the radionuclides produced in the reactor core. Once the fuel rods defect, fission product activity in the primary loops will increase and may lead to temporary shutdown of the reactor when fuel rod failure has deteriorated to certain levels. This paper introduces the theoretical mechanism of production and migration of fission product from defective fuel rods to the primary loops in the operating pressurized water reactors. The analytical solution of fission product concentration in primary loops is got based on the first-order differential equations for steady operation. Based on the release-to-birth ratios for recoil and diffusion by least squares method, a method is developed to diagnose the status of fuel failure in pressurized water reactors, including fuel failure time, the degrees of defect size, the disseminated actinides, the average burnup and the fuel batch. The prediction results of fuel failure are verified well with that of a typical fuel failure with secondary degradation in one commercial pressurized water reactor, as well as those of the sipping test and the post irradiation examination.