A lens module with adjustable magnification is installed in a scanner. The lens module can be used to collect and focus a scanning light beam to form an image onto at least one optical sensing device, which may be replaceable. The lens module includes a primary lens and at least one correction lens. The correction lens is with respect to the size of the optical sensing device, such as the total length of the effective pixels, and is installed at a location in front of the light collection end or between the image output end and the optical sensing device. This allows the size of the image focused by lens module to match with the size of the optical sensing device, so that the image can be captured by the optical sensing device.