Analytical thermal characteristics of current gain for BJT-BSIT (bipolar junction transistor-bipolar static induction transistor) compound device in the low current operation is proposed. The author makes the thermal model and also obtains a best thermal compensating factor of the compound device that indicates the relationship between the thermal rating of current gain with device structure parameters. It is important for the better design of compound device. At last, the analytical model is found in good agreement with numerical simulation and experimental results. The test results demonstrate that the thermal rating of current gain is less than 10% in 25degC-85degC and 20% in -55deg-25degC.