When pump depletion is considered, the analytical solution of Fokker-Planck equation of the nonlinear time-dependent degenerate optical parametric amplification (DOPA) in the P representation for generation of squeezed light is calculated. And then, the quantum fluctuation near or far away from threshold is evaluated. If the term (∝η) is neglected, the ordinary solution transits to the previous obtained linear approximate solution naturally. When considering the term (∝η), the solution and the perturbation series expansion theory of Drummond et al.. near threshold are coincided and adapted, while the perturbation series expansion theory far away from threshold is not adapted. The solution can apply to the whole region near or far away from threshold. When μ→0, quantum fluctuation is close to vacuum fluctuation; while μ1, the squeezing is near to the linear theory 1/1+μ.