The invention relates to a pipe-jacking starting-frame supporting system and method for constructing tunnel connecting channels. The supporting system comprises a stretchable-adjustment bottom jackingsupporting structure fixedly arranged at the bottom of a starting frame, a stretchable-adjustment top jacking supporting structure fixedly arranged at the top of the starting frame and a stretchable-adjustment lateral jacking supporting structure fixedly arranged on the side, away from a to-be-constructed connecting channel, on the starting frame; the bottom jacking supporting structure, the topjacking supporting structure and the lateral jacking supporting structure are jacked and supported into a main tunnel through stretchable adjustment of the bottom jacking supporting structure, the topjacking supporting structure and the lateral jacking supporting structure, and therefore the starting frame is supported into the main tunnel. The starting frame is assembled through the supporting system, assembling and disassembling are fast and convenient, the pipe-jacking starting-frame supporting system and method can better meet the requirements of functionality and structural stability ofnarrow-space tunnel connecting channels, the construction safety risk is reduced, and the construction efficiency is improved.