In this paper we study a optomechnical system enclosed by a optical cavity with one mirror attached to a spring as a closed quantum system. We provide a different sight of view of studying the backaction effect from the quantum field to the mirror. Instead of concentrating on the force exerted on the mirror, we focus on the energy transferring of the process, which encourages us to use the Feynman diagram method in weak coupling regime. We show that the observed photon frequency, mirror oscillation frequency and ground state energy are shifted as a result of emission and absorbtion of virtual particles. We emphasis that the integrals in the loop diagrams are convergent, which means by treating the motion of the mirror with quantum mechanics, we get finite result in energy transferring process. At last we use Feynman diagrams to give the transition amplitudes of general dynamical Casimir process.