A lens module and an assembling method thereof are provided. A lens set includes a barrel and at least one lens disposed inside the barrel. A first position-limiting member is connected to the barrel. A first hollow tube surrounds the barrel and has a first sliding path. The first position-limiting member is configured to slide along the first sliding path before the first hollow tube is fixed so that the barrel is capable of moving with respect to the first hollow tube before the first hollow tube is fixed. The first position-limiting member is limited by the first sliding path. A second hollow tube surrounds the first hollow tube and has a second sliding path. The first position-limiting member is configured to slide along the second sliding path before the first and second hollow tubes are fixed. The second sliding path is inclined to the first sliding path.