An Intra-cavity Continuous-Wave Optical Parametric Oscillator(IC-OPO) was developed in order to resolve problems inherent in IC-OPO such as instability of output power, long-lived bursts of relaxation-oscillations, etc. First, compensative design of thermal lens effect was devised to improve pattern matching. Second, an aperture slot was used to suppress the oscillations of high-order transverse mode. Finally, by placing a KTP crystal in the cavity for the purpose of SHG, the relaxation oscillation was suppressed by nonlinear loss. After these improvements, the compact tunable infrared to mid-infrared(2.2-4.2 μm) continuous source with high-stability was obtained. The amplitude stability of output power is 0.52%(RMS) and peak-to-peak amplitude stability is less than ±1.8%. The beam quality M2 is 1.72. Especially, a series of observations show that the relaxation-oscillation is suppressed effectively. Without KTP crystal, the threshold (808 nm) is as low as 1.5 W.