Abstract: By matching the length of the OPO cavity and the fundamental laser cavity, the Q-switched mod-locking intr-cavity Optical Parametric Oscillator (OPO) was realized. The design of fundamental laser and OPO cavity proved synchronously pumping and the Q-switched mod-locking signal pulse was experimentally obtained. The signal profile, output energy and spectroscopy were also measured in experiment. It is concluded that the repetition of signal mod-locking pulse depends on the fundamental and the depth of mode lock can reach 100% with 12.8 J pump energy and 20 kHz electr-optical (EO) repetition. The output energy of signal light depends on the pump energy and EO repetition.