Abstract: A new method for predicting the off-design performance and the surge margin of axial-flow compressors is presented, it employs a group of curves generalized from the experimental data of many subsonic and transonic rotors to determine stage stall points. For calculating the off-design performance of stages a more reasonable iterative method is adopted, in which the changes of wheel speed ratio and velocity ratio of rotor inlet and outlet, deviation angle and compressibility effects are taken into consideration. The overall compressor performance is obtained by stage-stacking method. The surge margin of the compressor is predicted by means of the "actuator-delay-volume" model and the method of numerical integration. As examples, the characteristics of two compressors are calculated. The results compare with the experimental data satisfactorily.