The generation and evolution of LiNbOa optical parametric oscillators synchronously pumped by frequency-doubled light from a mode-locked Nd:YAG laser has been studied by computer numerical simulation. It is shown that under the condition of pumping depletion the shapes and durations of the pulses in a signal wave pulse train are distinct from each other. The pulse duration in front of the train is shorter than that in the middle and at the end of the train(from 7 ps to about 20ps),meanwhile most of the pulses have substructures. Cavity length dismatch does not broaden the pulse very much but mainly reduces the output energy. The shape, the duration of pulses and the dismatch length of cavity are affected by the gain of the device.