Based on the theory developed by Safonov and Yamazaki in nonlinear radiation damping of parametrically excited spin waves (J. MMM. [STHZ]161[STBZ] (1996) 27 5), we calculate the effect of microwave cavity Q_value on the threshold of instab ility and autooscilations of microwave absorption, considering both processes of nonlinear magnon_magnon interaction and magnon_photon interaction. The resu lt is basically consistent with experimental data.