According to the operational principle of the Bipolar Junction Transistors(BJTs), comparison and optimization of two different mesa edge termination techniques on high-voltage 4H-SiC BJTs by a computer-based analysis are presented here, aiming at increasing the breakdown voltage. Based on the avalanche mechanism, the breakdown condition is first derived and then the breakdown voltage is numerically calculated. The calculation results show that various parameters including doping concentration, implantation depth and the distance from the base mesa can have great influence on the high-voltage performance.