Self recirculation casing treatment has been showed to be an effective technique to extend the flow range of the compressor. However, the mechanism of its surge extension on turbocharger compressor is less understood. Investigation and comparison of internal flow filed will help to understand its impact on the compressor performance. In present study, experimentally validated CFD analysis was employed to study the mechanism of surge extension on the turbocharger compressor.Firstly a turbocharger compressor with replaceable inserts near the shroud of the impeller inlet was designed so that the overall performance of the compressor with and without self recirculation casing treatment could be tested and compared. Two different self recirculation casing treatments had been tested: one is conventional self recirculation casing treatment and the other one has deswirl vanes inside the casing treatment passage. Then Numerical simulation was also performed and the result shows that CFD was able to predict the performance of the compressor with and without self recirculation casing treatment very well. Both numerical and experimental results show that casing treatment with deswirl vanes could get about 20% of more surge margin compared to the one without deswirl vanes inside the casing treatment passage. Numerical analysis indicates that suppression of the tip leakage moving toward the impeller inlet is the main mechanism of the surge margin extension of self recirculation casing treatment. Deswirl vanes inside the casing treatment passage could reduce the positive swirl of the recirculation flow and the pressure rise of impeller could be increased compared to convention self recirculation casing treatment, which helps to further extend the surge margin.