Abstract: Amplified spontaneous emission(ASE) and parasitic oscillation in laser resonators exhibit distinct physical properties. An analytic method is employed to find eigenmodes of disk medium. These modes processes some non-trivial dispersion relations, which allow few stable longitudinal modes only. It decreases amplifying rate of parasitic oscillation in mode competition with main laser. A numerical model is further proposed to perform (3+1) dimensional simulations. Ray tracing method, together with finite-difference in time domain and Fox-Li type iteration, are used to solve a set of coupled equations. Transverse mode competition between ASE/parasitic oscillations and main laser increases threshold to establish laser oscillation and decreases output power as well. Time evolution of laser outpu