The present invention provides an optical sight with rangefinder and assembly method thereof. The optical sight comprises an erector lens unit, a photoelectric rangefinding unit, a barrel, an objective lens unit, a plurality of adjusting elements, a power supply unit and an eyepiece lens unit disposed on the barrel. The photoelectric rangefinding unit includes a laser emitter, a laser receiver and a range indicator. A plurality of openings are formed on sidewall of the barrel corresponding to the positions of the laser emitter, laser receiver and range indicator for conducting the cavity to outside the barrel. The assembly method comprises the steps of: assembling the erector lens unit, the photoelectric rangefinding unit and the barrel together, then connecting an objective lens unit and mounting a plurality of adjusting elements, adjusting the objective lens unit and mounting the adjusting elements and the power supply unit, adjusting the focus of the laser emitter and the laser receiver and adjusting the position of the range indicator through the openings formed on the barrel for complying with the requirement of photoelectric rangefinding performance. The assembly method of the optical sight according to the present invention can be adjusted after all elements are assembled. The assembly method of the optical sight according to the present invention can be used to reduce the adjusting times and prevent unnecessary disassemblies and re-assemblies for simplifying the assembly process and improving the yield rate and the productibility of the optical sight.