A systematic theory of quantum fluctuation in the time-dependent linearly driven parametric amplification is developed. At first, a time-dependent linearly driven Fokker-Planck equation for the degenerate optical parametric amplification system is deduced when the pump depletion is considered, and then the quantum fluctuation below or near the threshold is evaluated, which is in agreement with that obtained by the linear theory or perturbation expansion near the threshold. Above the threshold, the short-time behavior of our solution is close to the linearization approximation; however, with the increase of interaction time τ, the long-time behavior of our solution shows that the squeezing is quite different from the linear theory.