Based on the theory developed by Safonov and Yamazaki in nonlinear radiation damping of parametrically excited spin waves (J. MMM. STHZ161STBZ (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.