The invention discloses a construction method for an opening of a contact channel of an existing shield tunnel, which comprises the following steps of: firstly, supporting and reinforcing duct pieces which do not occupy a road in an opening area and an influence range by adopting a duct piece supporting and reinforcing device; then a three-pilot-hole three-step construction method is adopted for duct piece opening construction, specifically, a middle pilot hole is excavated firstly, and a middle pilot hole connector ring beam, a stand column and a temporary steel support are constructed; a left pilot tunnel is constructed, and the left side ring beam is closed; finally, a right pilot tunnel is constructed, the right side ring beam is closed, and at the moment, totally-closed construction of the opening area ring beam is completed; and dismantling the temporary support after the ring beam is closed and reaches the design strength. According to the method, construction treatment of the existing shield tunnel evacuation channel and the entrance and exit duct piece opening can be safely and rapidly completed, the construction progress is fast, structural deformation is small, the safety quality is guaranteed, the treatment period is effectively shortened, it is ensured that a set target of a construction unit is smoothly achieved, and social benefits and economic benefits are remarkable.