The invention discloses a construction method for an underground excavation tunnel side-direction hole opening supporting system, and belongs to the field of tunnel engineering. Underground excavation tunnel side-direction hole opening is achieved after an initial lining of underground excavation tunnel construction is completed. According to initial lining construction of an underground excavation tunnel side wall hole opening portion and a subvertical shaft excavation portion, suitable external expansion needs to be carried out according to a ring beam structural size determined in advance, determination of the ring beam structural size of the supporting system is the key point of the construction method, meanwhile, construction quality is ensured, and the construction key point is that the initial lining of underground excavation construction in the construction process is effectively connected with the supporting system to form whole supporting closing; the key point in the construction process is that the structure and personnel security in the construction process is ensured, and the supporting system is constructed in advance; a three-dimensional cage-type supporting system composed of an annular beam and a linkage beam is adopted, meanwhile, a steel plate needs to be embedded at the corresponding portion of the supporting system, and the initial limiting inside a hole in the hole opening construction process is connected with the supporting system.