A new analytical solution for the phase_mismatched Fokker_Planck equation of deg enerate parametric amplification, and its application in evaluation of the ampli tude quantum fluctuation after passing through the quasi_phase_matching(QPM) dev ice is presented. The calculated results for QPM device, agree with that of the Langevin equation in the case of no loss k=0, and give a general solution fo r k≠0, from which we can derive the influence of squeezing on the loss coef ficient k.