The composition and the working principle of the test system for the sapphire optical fiber temperature sensor were analyzed. As there is a big temperature difference between the inner and outside surface of black body cavity, so the error of temperature of the measured object is presented. To solve this problem, a process of separating variables was used to derive the outer surface temperature of black body cavity from the inner surface temperature of the blackbody cavity, which further improved the performance of the sensor required by test. The experiment tested the feasibility of this method, and the measurement accuracy of the sapphire optical fiber blackbody cavity temperature sensor was improved.