Reflex light-tubes are widely used in optoelectronic measuring systems such as in ships, vehicles and so on. To make the optical path reflex in altitude direction and keep the incident light parallel to the emergent light in azimuth transmission, the reflex light-tubes are needed, which finally ensures precision transmission of the azimuth reference. The flat mirror method and the mercury method are used to test the optical-axis parallelism of reflex light-tubes shorter than 1 m, however when the methods are applied to reflex light-tubes longer than 3 m, the accuracy required cannot be reached. A new test method of optical-axis parallelism for three-meter reflex light-tube is introduced and it is well suited for testing the parallelization of reflex light-tubes more than 3 m.