Passive intermodulation (PIM) is a complex problem in high-power microwave devices and satellite communications. In this paper, an effective calculation method is proposed for predicting PIM power levels of the cascaded microwave devices. First of all, the analytical formula of intermodulation voltage is derived based on the nonlinear I–V characteristics of microwave devices. Then, the mathematical model of point sources is constructed by the transmission line theory and extended to the cascaded microwave devices. The passive intermodulation products (PIMP) of the cascaded microwave devices are evaluated based on the point-source model. The relationship of PIM between a single microwave device and the cascaded system is revealed. Eventually, the corresponding experiments are designed to verify the accuracy of point-source model and the cascaded model to predict the third-order PIM power level, which address the problem of PIM prediction of the cascaded microwave devices.