The invention relates to a method for preparing a polycrystalline silicon thin film, in particular to a method for inducing an amorphous silicon thin film with tin to be crystallized into a polycrystalline silicon thin film at a low temperature. according to the main technical scheme of the invention, the method comprises the following steps of: 1, carrying out ultrasonic cleaning on a glass substrate by using acetone and deionized water respectively to clean the glass substrate; 2, growing a layer of amorphous silicon (a-Si:H) thin film on the glass substrate by using a plasma enhanced chemical vapor deposition method (PECVD), wherein the temperature of the substrate is about 200 DEG C, and the thickness of the thin film is 200nm-300nm during deposition; 3, growing a layer of metal tin (Sn) thin film with the thickness of 10nm-20nm on the amorphous silicon thin film by using a physical vapor deposition method so as to obtain a substrate/(a-Si:H)/Sn structure; 4, arranging the substrate/(a-Si:H)/Sn in a heat treatment furnace in a laminated manner, and carrying out heat treatment for more than 1 hour at the temperature of 450 DEG C, then cooling naturally, where nitrogen (N2) is introduced and used as a protection gas during the whole annealing process; and 5, removing the remained metal tin on the surface with concentrated hydrochloric acid with the concentration of 37.5% to finally obtain the polycrystalline silicon thin film which is crystallized under the induction of tin, wherein the particle size is about 70-200nm. The method is applicable to the fields of thin-film field effect transistors and solar cells.