The spontaneous radiation of single photon in optomechanical system is investigated by using the microscopic master equation and damping bases method. A mechanical vibration mode and an optical cavity mode are coupled through radiation pressure in typical optomechanical system. Compared with the quantum optical master equation, microscopic master equation can describe the decay of energy eigen-state and deal with the problems related to the decay process more accurately. In the study of time evolution of the single photon spontaneous emission process, the damping bases method is used to solve the microscopic master equation. The mean photon number calculation results of cavity mode show that the probability-superposition of decay process of excited states determines the spontaneous radiation of single photon in optomechanical system. In addition, the microscopic master equation can also be used to deal with some related problems in ultrastrong coupling optomechanics.