For EV battery safety and power performance during running, this paper combines discrete Fréchet distance method and identification of battery charge-discharge curve to propose a diagnostic method of battery SOH based on online test platform. System can control directly EV battery to complete the full charge-discharge test online by CAN communication module, and get the actual operational status data. EV service full cycle information databases are established to forecast the diagnosis of health degree, which can guide power battery safety assessment and balanced maintenance. By analyzing the cases of test calculation, the correctness and accuracy of the method is verified after comparing diagnosis result and test data of switching off.