A tunable continuous-wave(CW) internal cavity pump optical parametric oscillator(OPO) based on multi poled period MgO∶PPLN was reported. The 3.2~4.1 μm wide tuning mid-infrared parametric light was obtained in real time by changing the poled periods and temperature. The thermal effect and spot modes matching during OPO′s wide tuning process were considered. It used a folded type doubly cavities which consisted of 1.064 μm resonator and optical parametric oscillator. The optimum parameters of the cavity structure was determined on account of the numerical simulation and theoretical analysis results. The theoretical simulation of tuning characteristics of OPO was carried out. Based on this structure, the effect of temperature and polarization period on output wavelength was experimentally investigated. It overcame the sever absorption and the quantum loss of MgO∶PPLN in the 3.8~4.1 μm mid-infrared band. Continuous wide range tunable output of 2.78~4.18 μm in mid-infrared radiation was achieved. The tuning range was up to 1.399 μm. The output power of 3.2 μm, 3.5 μm, 3.8 μm and 4.1 μm parametric light was 1.72 W, 1.39 W, 0.79 W and 0.442 W, the corresponding conversion efficiency was 7.17%, 5.4%, 3.1%, 1.84% and 1.72 W.