In a molten salt reactor, the volume change in the primary loop may lead to an overflow of fuel salt, which will challenge the radioactive containment. Several factors, such as the thermal expansion, graphite dimensional change caused by fast neutron irradiation, fuel feed online and fuel salt reconstitution that affect volume change are analyzed based on a designed small modular thorium-based molten salt reactor (SM-TMSR) in this paper. The results show that the thermal expansion of nickel alloy is comparable to that of fuel salt, the net volume change due to isothermal expansion is not obvious, about 0.04 m3 per 100 °C in SM-TMSR, which can be easily contained in a pump bowl without an additional overflow tank. Furthermore, the volume decrement due to graphite shrink by irradiation (based on the IG-110 data) is more obvious than the volume increment by fuel feed online, which will lead to a decline of liquid level during the long-term operation. 0.13 m3 excess volume of fuel salt should be provided in the pump of SM-TMSR. Finally, as we analyzed, fuel salt reconstitution will add to the difficulties in volume control, reactivity control and the control of heavy metal mole ratio in fuel salt. It is recommended that the used fuel salt can be recycled into a new power module without any treatment, and the increased volume of fuel salt caused by feed online in last power module can be accommodated in a larger downcomer.