  This paper investigates an averaging principle for stochastic Klein-Gordon
equation with a fast oscillation arising as the solution of a stochastic
reaction-diffusion equation evolving with respect to the fast time. Stochastic
averaging principle is a powerful tool for studying qualitative analysis of
stochastic dynamical systems with different time-scales. To be more precise,
the well-posedness of mild solutions of the stochastic hyperbolic-parabolic
equations is firstly established by applying the fixed point theorem and the
cut-off technique. Then, under suitable conditions, we prove that there is a
limit process in which the fast varying process is averaged out and the limit
process which takes the form of the stochastic Klein-Gordon equation is an
average with respect to the stationary measure of the fast varying process.
Finally, by using the Khasminskii technique we can obtain the rate of strong
convergence for the slow component towards the solution of the averaged
equation.
