A study of boron impurities deposited on a Si(100) surface in a SiHCl₃-BCl₃-H₂ system is reported in this paper, using periodic density functional theory with generalized gradient approximation (GGA). The results show that the discrete distances of BCl₃ and SiHCl₃ from the surface of the Si(100) unit cell are 1.873 Å and 2.340 Å, respectively, and the separation energies are −35.2549 kcal/mol and −10.64 kcal/mol, respectively. BCl₃ and SiHCl₃ are mainly adsorbed on the surface of the Si(100) unit cell in particular molecular orientations: the positive position and the hydrogen bottom-two-front position from the analysis of the bond length change and adsorption energy. The adsorption of SiHCl₃ and BCl₃ is accompanied by a charge transfer from the molecule to the surface of the unit cell of 0.24 and 0.29 eV, respectively. BCl₃ reacts more readily than SiHCl₃ with the Si(100) surface, resulting in the deposition of boron impurities on the polysilicon surface.