The effects of a series of parameters such as injected signal intensity , pump intensity, crystal length have been analyzed in Optical parametric amplification(OPA). The gain saturation and pump depletion were taken into account. The stability of an OPA system was investigated in both the small signal injection and the large signal injection. Experimental investigations of OPA based on two noncollinear phase-matched LBO OPA under the small signal injection and the large signal injection have been carried out，and the experimental data are in agreement with the theoretical results. The theoretical and experimental results all demonstrated that there exists a strong gain saturation region where parametric light with high efficiency and high quality and high stability can be obtained by properly choosing relevant system parameters.