The mechanism of compressor stall margin enhancement using the tip air injection is explored. The transonic compressor, NASA Rotor 37, is taken as the object to study the tip clearance flow under active control of tip air injection by numerical simulations. The effects of injection parameters (injection total temperature, injection position, injection angle, injection mass flow, injection port size, injection type and etc) on the stall margin extension are emphatically analyzed. Results show that the enhancement of tip leakage vortex enlarges the low-energy region induced by the shock wave in the row channel when the working condition is moving to stall point. In addition, the enhancement of radial vortex increases its entrainment ability, which tends to expand separation zone. Once the tip injection imposed, the decrease of the leakage vortex intensity widens the stall margin, while the total pressure loss increases to some extent due to the mixing of the tip micro jet with the mainstream. It is found that injection parameters should be restricted to a moderate region so as to achieve a good stall margin extension without an excessive increase in the pressure loss .