The geometric, electronic, and magnetic properties of Ho-doped Sin (n=1-12, 16, and 18) clusters are studied via the density functional theory with the generalized gradient approximation. The results show that the Ho atom in the most stable HoSin clusters always occupies the surface site. Starting from n=18, the Ho atom abruptly drops into the center of Si frame, forming the Ho-encapsulated Si cages. The stabilities of HoSin clusters increase with increasing size n. The Ho atom in HoSin clusters enhances their chemical activities. Moreover, the magnetisms of HoSin clusters are independent of their geometric structures, and the Si and Ho atoms are antiferromagnetic.