  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.
