In noncritical phase matching condition, it is necessary to precisely control the stability and uniformity of the crystal temperature in full aperture. A device using resistive heater to control the temperature of the large aperture crystal was introduced. Taking full account of the low thermal conductivity and large aperture of the crystal, it was heated by thermal conduction and the natural convection of the inner air, which was heated up at the same time for temperature balancing. The characteristics of temperature distribution of the device were studied. Through experiments and simulations analysis, it was found that the crystal temperature of steady state and the necessary heating time varied with the heating length and the thermal conductivity of the crystal material. The result shows the practicability of the thermal control of the crystal with the aperture of Φ80 mm. The temperature difference of the crystal, in steady state, could be controlled within ±0.15°C.