With the wide use of magnesia heating cables in various industries, understanding their insulation characteristics under different temperature and humidity conditions has become critical. In this paper, the changes of voltage and current of magnesia heating cable are measured by increasing the temperature step by step and changing the voltage boost mode. In terms of humidity, the principle of single variable is followed, and the insulation performance is analyzed by changing the relative humidity and humidification time. The experimental results show that the voltage and current of MGO heating cable are affected by the different voltage boosting modes under high temperature conditions. The increase of humidity leads to the decrease of cable insulation characteristics. Nevertheless, the cable was able to maintain its integrity throughout the test range without breaking down.