The invention relates to a test apparatus for the insulation characteristic of mixed gas. The test apparatus comprises a seal box, an electrode pair disposed in the center in the seal box, and a temperature control system for adjusting the temperature inside the seal box. The seal box is equipped with gas charge openings for gas to be tested. The seal box is provided with an air gauge. The invention also provides a test method for the insulation characteristic of mixed gas, which applies the above test apparatus. The test apparatus and method are used for test the insulation characteristic of mixed gas in high/low temperature and high/low pressure conditions. Therefore, the optimal ratio of mixed gas and the optimal matching pressure of gas insulated equipment in different temperature ranges are obtained quickly and conveniently, and a GIS gas insulation design scheme in cold areas with high altitude is optimized.