Abstract: Miniaturization of nuclear energy systems will be a promising direction for nuclear industry development in the future, in which micro lead-cooled fast reactors (LFRs) are a type of technical route in relevant. During the R&D of micro LFRs, transient thermal-hydraulic safety analysis is one of the important research fields, which can not only verify the safety of the design, but also provide optimization reference for the follow-up design of the reactor. SMILE is a megawatt-level micro LFR with high safety and compactness, designed by State Power Investment Corporation Research Institute. For SMILE, pure lead is applied as the coolant. The fuel adopts UO₂, while the cladding and structural materials use independently developed wide oxygen-controlling corrosion-resistant materials, which can simplify or even omit complex oxygen measurement and control systems. Its core is designed for long-life without refueling, eliminating complex refueling devices. Its reactivity is controlled by external drums. The main system employs a compact design scheme, using a pool-type integrated compact layout, with an integrated main pump and main heat exchanger structure. The secondary system uses a compact water steam cycle or S-CO₂ Brayton cycle, along with a non-passive residual heat removal system. Due to the advanced concepts of lead-cooled fast reactor designs, there has not been a mature unified thermal-hydraulic safety criterion. Based on SMILE’s design characteristics and a literature review, some key parameters’ performance criteria for thermal-hydraulic safety are proposed, including flow velocity of the coolant, coolant temperature, temperature of the fuel cladding, etc. During reactor operation, various abnormal events deviating from normal operating ranges may occur, potentially leading to accident conditions. For SMILE, unprotected loss of flow (ULOF), unprotected transient over-power (UTOP), unprotected loss of heat sink (ULOHS), were deeply investigated in the present paper. ATHLET (analysis of thermal-hydraulics of leaks and transients) is a best-estimate one-dimensional system safety analysis program developed by Gesellschaft für Anlagen-und Reaktorsicherheit (GRS), which is applicable to analyzing various reactor types such as pressurized water reactors, boiling water reactors, graphite-water-cooled reactors, and CANDU reactors, for both benchmark and beyond-benchmark accident analyses. With the development of Gen-Ⅳ nuclear energy systems, GRS has extended the applicability of ATHLET, incorporating properties of metals and liquid metal heat transfer relationships, enabling to be applied for analyzing the thermal-hydraulic characteristics of lead-cooled fast reactors. Through analysis, it is found that under normal conditions, the results obtained from ATHLET are in good agreement with the design values, thus AHTLET program is capable of analyzing the transient features of SMILE. Under typical unprotected transient accident conditions, SMILE shows a good natural circulation performance and thermal characteristics, in particular given it inherent safety features. However, according to simulation results, in the event of an ULOF transient accident, the cladding temperature of the fuel exceeds the maximum temperature limit, which could potentially threaten the reactor’s safe operation. Therefore, in the future high-temperature corrosion-resistant materials will be developed, for eliminating or mitigating its impact on the system’s safe operation.