The ACP100 is a small modular reactor with integrated passive design, and the best estimate plus uncertainty method is utilized to conduct a realistic analysis of the loss of coolant accident caused by a double-ended direct vessel injection line break of the ACP100. The thermal-hydraulic system code ARSAC is used, and the figure of merit for reactor safety is determined to be the minimum collapsed core level. Subsequently, important phenomena which may affect the core level are identified, and uncertainty distributions of the inputs are quantified, followed by the uncertainty propagation and sensitivity analysis. The tolerance limit of the minimum collapsed core level is obtained, and the key parameters during different accident phases are identified. According to the results, parameters related to the flow resistance of the injected coolant and some constitutive models of the ARSAC can be optimized to further deepen the reliability and safety design of the ACP100.