Abstract: The continuous-wave (CW) mode-locking laser pumping intra-cavity optical parametric oscillator (OPO) was realized with experiment, output of mid-infrared idle light was gotten. The cavity shape, position of cavity mirror, cavity length were determined by ABCD matrix considering stable cavity. By matching the length of the OPO cavity and the fundamental frequency laser cavity to meet pumping condition synchronously. Firstly, the output profile of SESAM mode-locking laser was measured without OPO oscillating, the threshold of stable continuous-wave mode-locking running was found. Secondly, output power, output waveform, repetition frequency, and pulse width of the idle frequency, signal and fundamental frequency light of intra-cavity OPO were also measured. It is concluded that the central wavelength of the idle frequency light is about 3.3m through measuring spectrum of signal light and fundamental frequency light.