Abstract: Thin convex lenses are the most important and basic components of optical instruments. It plays an important role in many fields such as astronomy, military and medicine. The focal length is the most important characteristic parameter of optical systems such as thin lens and mirrors. Hence, it is particularly important to accurately measure the focal length of the thin lens. The methods for measuring focal length of thin convex lens are the object/image distance method, self-collimation method, optical-electrical method and parallel tube method in laboratory. Aiming at the problem of low accuracy of measuring the lens focal length by the first three methods, the parallel tube method was adopted to measure the focal length of thin convex lens and the experimental error was analyzed simply. Experimental result shows that, the method can measure the focal length of thin convex lens with high precision, the relative error is only 0.138 %. Therefore, the method for measuring focal length of thin convex lens by parallel tube method is effective and feasible.