Surface passivation of crystalline silicon is the key technology of high efficiency crystalline silicon solar cells, which directly affects the performance of crystalline silicon devices. In this paper, the surface passivation of crystalline silicon (001) by a superacid bis(trifluoromethane)sulfonimide (TFSI) was studied by first principles method. It is found that the four oxygen atoms of TFSI can effectively bond with Si atoms on Si (001) surface, and the adsorption energy is -5.124 eV. The electron localization function shows that the bonding type between O atom of TFSI and Si on the surface of crystalline silicon is metal bond. The analysis of density of states and charge difference density show that the electrons on the Si surface are transferred to TFSI, which effectively reduces the electron recombination on the Si surface and improves the minority carrier lifetime of crystalline silicon. Bader charge shows that the charge of Si atoms on the surface decreases with the passivation of Si atoms on the surface of crystalline silicon by TFSI, while the charge of O atoms and S atoms in TFSI increases, which further proves the electron transfer after TFSI passivation of Si surface. This work provides data support for the first principle method to predict the passivation effect of organic strong acid passivated crystalline silicon surface.