Thorium fission reaction rate is an important datum in uranium-thorium fuel cycle. In order to measure the thorium fission rate on the thorium sample equipment which is set up by the conceptual design of the subcritical reactor and to check the thorium data, the off-line activation γ measurement method of thorium fission rate is developed. Combined with thorium fission yield data of 85mKr, the 232Th fission reaction rate distribution in thorium sample device can be obtained by measuring the 151.16 keV feature gamma rays emitted by fission fragment 85mKr. Details of the principles and factors of this method are discussed, and the verification experiment is carried out on a depleted uranium shell of R13.1/30.0 cm with D-T neutrons. The relative uncertainty of experiment is 5.3%-5.5% for thorium fission reaction rates. The experiment is simulated using MCNP5 with ENDF/B-VI and ENDF/B-VII libraries, simulation results and experimental results accord well with each other within the experimental uncertainty, showing that this approach developed in this paper can work well for determining the thorium fission rate.