Dynamical behaviours of a single-mode laser Haken-Lorenz system were investigated.The steady state solution of the system was obtained. Stability analysis was given by using the principle of linear stability. Characteristic equation was gotten and Hopf bifurcation point μ_c was determined. A periodic oscillation analytic solution for the system at bifurcation point was obtained by using the method of series. The dynamical behaviors of the system were given by computer numerical simulation. The results show that when the bifurcation parameterμ crosses the critical value μ_c and Haken-Lorenz system gives rise to limit cycle,i. e. periodic oscillation solution,the result accords with the analytic one.