A method for assembling a lens or a lens unit is determined according to the design result and tolerance of an optical system in which an infrared lens is used. The mount and barrel of the lens are designed specially. The center error of the lens unit is measured by an OptiCentric dual-path centrescope. After the lens unit is positioned with pins, each lens unit is reset with pins. When the whole lens unit is assembled, the image quality of the lens is detected in an infrared Modulation Transfer Function (MTF) measurement instrument. The result shows that both the effective focal length and the MTF of the infrared lens unit meet the required specifications and this new dual-path assembling method is reasonable and feasible.