The thorium-uranium (Th-U) fuel cycle is considered as a potential approach to ensure a long-term supply of nuclear fuel. Small modular molten salt reactor (SMMSR) is regarded as one of the candidate reactors for Th utilization, since it inherits the merits of both MSR and small modular reactor. The Th utilization in a 220-MWe SMMSR with the once-through fuel cycle mode is investigated first. Then, the SMMSR with batch and online fuel processing modes is investigated second for comparison, considering the progressive development of fuel reprocessing technology. To keep a negative temperature reactivity feedback coefficient (TRC), a configuration for fuel salt volume fraction (SVF) equal to 15%, with a mixed fuel of low enriched uranium (LEU) and thorium at an operation time of 5 years is recommended for the once-through mode, corresponding to the Th energy contribution (ThEC) of 37.6% and natural U and Th utilization efficiency (UE) of 0.51%. Considering the solubility limit of heavy nuclide (HN) proportion (below 18.0 mol%) in the fuel salt, the total operation time of the SMMSR shall be less than 50 years for the batch reprocessing mode with a 5-year reprocessing interval time. In this case, the ThEC and UE can be improved to about 47.4% and 0.99%, respectively. Finally, the Th utilization and fuel sustainability are analyzed at a lifetime of 50 years for the online reprocessing fuel cycle mode, including both the only online fission products (FPs) removing scheme and the fuel transition scheme from LEU to U-233. For the former scheme, the ThEC and UE can be further improved to 58.6% and 1.52%, respectively. For the latter scheme, Pa-233 is extracted continuously from the core to breed and store U-233. If a total reactor lifetime of 50 years is assumed, the operation time using LEU as starting and feeding fuel for 6 years is required, and the bred U-233 during this 6-year operation can start and maintain the reactor criticality for the remaining 44 years. In this case, the ThEC is improved significantly to 89.1% corresponding to a UE of 2.74%.