A method for assembling a lens module includes the following steps. Firstly, a lens barrel, at least one optical element, a spacer ring, and a fixing ring are provided. The spacer ring includes an annular projection along a circumference of the spacer ring. The fixing ring includes a annular pipe and glue received in the pipe. The film is made of ethylene-vinyl acetate copolymer. Secondly, the at least one optical element and the spacer ring are put into the barrel, with an annular groove defined between the projection and the barrel. Thirdly, the fixing ring is placed in the groove. Fourthly, the fixing ring is heated and the film is melted to let the glue fill the groove. Finally, the glue is cured to fix the spacer ring and the at least one optical element in the barrel.