Abstract: The small break loss of coolant accident（SBLOCA） experiment of surge line was performed on the small modular reactor passive safety system test facility, to investigate the thermal hydraulic phenomena and the performance of passive safety system during the accident. In the experiment, the fluid density difference between the pressure balance line and injection line driven the coolant to inject into the Reactor Pressure Vessel（RPV）. When the pressure balance line is no longer submersed, the CMT injection flowrate became unstable; The Accumulator（ACC） had prominent effect on the cooling of the core during the initial period of accident; After the sufficient depressurization of the Automatic Depressurization System（ADS）, the coolant in the In-containment Refueling Water Storage Tank（IRWST） provided sustaining and stable injection and cooling of the core. The experimental results show that the passive safety system can provide effective coolant injection and successful removal of core decay heat under surge line SBLOCA.