The invention belongs to the technical field of mass flow rate measurement of nuclear reactors, and particularly relates to a device and a method for measuring the mass flow rate of small pores in a reactor. The device for measuring the mass flow rate of small pores in the reactor comprises a high-position container, a simulated orifice plate box body, a low-position container and a mass measuringinstrument. Liquid in the high-position container flows into the simulated orifice plate box body, and then flows into the low-position container through a number of small through holes used for simulating small pores in the nuclear reactor, wherein the small through holes are in the lower end surface of the simulated orifice plate box body. The mass measuring instrument measures the mass of theliquid accommodated in the low-position container. According to the invention, the mass flow rate of small pores in the nuclear reactor is accurately measured by calculating the mass flow rate of thesimulated orifice plate box body under a certain pressure difference, and the device and the method have an important significance for ensuring the working lives of relevant components and the safe operation of the nuclear reactor.