The oscillation mechanism of dual-wavelength Nd:YAG Q-switched laser with two-mirror resonator is studied on the basis of the rate equation. The characteristics of the dual-wavelength laser are discussed by analysing the competition process with different transmissivities. Results show that the oscillating time of the Q-switched pulse laser for 1319nm laser is delayed relative to that of 1064nm. The 1319nm Q-switched laser is formed finally from the first relaxation oscillation spike after the Q-switch opening, i.e. it restrains the rest relaxation oscillation spikes. The transmissivity of relatively balanced output energy is theoretically obtained.