Abstract: In this paper, the passive injection system operation characteristics of an integrated modular small reactor under different break sizes of direct vessel injection line（DVI） are studied by experiments. The test result shows that the bigger the safety injection pipeline break area is, the bigger the difference injection flow rate of the break side and non beak side core makeup tank system is. The accumulator injection has obvious effect on the core makeup tank injection in the case of large break area. In the early operation stage of the core makeup tank system, the operation mode is water-water cycle. After the steam flow into the pressure balance pipeline, the operation mode is steam-water cycle. The accumulator is driven by nitrogen expansion, and the injection flow rate is affected by the pressure drop rate of the system.