Abstract: The spent fuel was radioactivity.Its matrix was complicated and the content of tin was low.So measurement of tin in spent fuel was difficult. ¹²¹Sn and ¹²⁶Sn in spent nuclear fuel element were determined by isotope dilution-thermal ionization mass spectrometry(ID-TIMS) using natural standard tin solution as spike. The sample loading techniques of silica gel-tin-phosphate was employed. Ionization filaments heating rate and temperature holding time was optimized. Thirteen signals are collected by retarding potential quadrupole (RPQ), which effectively eliminating isobaric interferences of Te,Sb and CaPO₃⁺. The content and isotopic abundance of ng/g level of Sn is obtained.The mass concentration of ¹²¹Sn and ¹²⁶Sn in spent fuel are 5.5 ng/g and 230.5 ng/g, measurement uncertainty are 4.2% and 1.4%(k=2), respectively.