Abstract: Corrosion of cladding and structural materials by liquid lead or LBE is one of the key issues restricting the development of lead-cold fast reactor. A primary cooling system analytical model of 100MW_th small modular natural circulation lead-cooled fast reactor named SNCLFR-100 is established with ATHLET code, and the unprotected transient of partial flow blockage of the hottest fuel assembly was analyzed. The results of analysis indicate that the mass flow rate of the hottest fuel assembly will drop to about 50% of the rated value when the blocking rate β reaches 0.6, while the maximum temperature of the hottest pin cladding will reach 650℃. When β reaches 0.9, the mass flow rate of the hottest fuel assembly will fall to about 12.6% of the rated value, and the maximum temperature of the hottest pin cladding will exceed the cladding material melting point 1400℃, and cladding melting will occur in the hottest fuel assembly.