This paper presents the design and simulation of energy harvesting system based on radioisotope. This energy generator is a typical spring-mass-damping system that converts nuclear energy into mechanical vibration energy. The vibration energy is converted into extricable electrical energy by a capacitive energy converter which consists of a top Pt electrode and a bottom Al electrode deposited on vibration mass. The two parallel electrodes with different work functions will generate a build-in voltage for initially charging the capacitor. A physical model is established. The simulation results show that the maximum average output power of the simple external load is 0.16 μW, when the surface power of the radioisotope is 1.28 μW. The energy conversion efficiency is 12.5%.