Abstract: In order to protect the key devices rapidly and effectively from unexpected damage, a reliable interlock system was required and developed for soft X-ray free electron laser (SXFEL) in Shanghai. The distributed interlock system was designed by programmable logic controller (PLC) and experimental physics and industrial control system (EPICS). The functions of management and switch among several modes were employed in the interlock system by the design way of Master station plus local sub stations mode, so as to meet different demands of commissioning and running of the accelerator. All the functions of the interlock system were verified in the operation. The interlock system can ensure the accelerator work safely.