Permanent magnet synchronous motor is widely used in industrial sites. In some cases,it is started softly by variable frequency converter and then switched to operate normally with power frequency. In order to analyze such motor operating characteristic with voltage sag,a method for evaluating the low voltage ride through capability of permanent magnet synchronous motors is presented in this article. Based on the dq coordinate system,the analytical formula among electromagnetic torque,power angle and terminal voltage is established. With the relationship between slip and motor stability in the transient process,the slip based motor transient stability analysis criterion is established. It can be used to determine the safety and stability critical voltage of the permanent magnet synchronous motor. The analytical formula of critical cutting power angle is deduced by area law. Then,the critical cut-off time of fault can be got. Critical voltage and fault removal time is selected as the characteristic value to evaluate the motor's low voltage ride through capability. The transient stability can be judged by the negative slip value in the transient process. The smaller the motor load rate,the stronger the low voltage ride through capability. Simulation results verify the effectiveness of the evaluation method.