Piezoelectric energy harvesting based on circular plates has great potential in replacing chemical battery to provide power for low-power electronic devices. This paper investigates the harvesting performance of a piezoelectric circular plate energy harvester with a proof mass considering its contact area via theoretical modeling and numerical simulations. Firstly, based on the Kirchhoff thin plate theory, the generalized Hamilton principle is applied to derive the electromechanical coupling equations of the piezoelectric circular plate energy harvester considering the proof mass. The equations are approximately discretized using the Galerkin method, and the discretized equations yield closed solutions of voltage, power output, and optimal load resistance. In addition, the correctness of the theoretical model is verified via the finite element simulations, and the results show that the theoretical model can successfully predict the power output of the piezoelectric circular energy harvester. Finally, the closed solutions are used to explore the effects of related parameters, such as the load resistance, the proof mass, the inner and the outer radius of the piezoelectric circular plate, on the natural frequencies, the output voltage and power of the piezoelectric circular energy harvester. The results show that when the contact radius between the proof mass and the piezoelectric composite plate is small enough (here, the contact radius is less than 1/14 of the plate radius), the contact area between the proof mass and the circular plate can be ignored. It is found that the piezoelectric plate with inner diameter between 2.5 mm and 4 mm can improve the harvesting performance of the energy harvester, compared with the piezoelectric plate without hole. As well as that the proper choice of the mass, the load resistance and the outer radius of piezoelectric circular plate can not only reduce the natural frequencies of the piezoelectric circular plate, but also improve its harvesting performance.