The invention discloses a drilling and blasting construction method for mechanized full-cross-section quick excavation and ring formation of a weak surrounding rock tunnel. The construction method comprises the following steps of 1, advance pre-reinforcement of a tunnel face; 2, excavation of the tunnel face, wherein the tunnel face is subjected to mechanized drilling, powder charging and blastingexcavation, during drilling, except an inverted arch, the tunnel face is drilled in a mode of straight hole blasting, the inverted arch is drilled through a technology of postposition-free inverted arch primary support and bottom hole and slant hole blasting, bottom hole and slant hole drilling is downwards conducted in the tunneling direction, the mode of combining long and short holes is adopted, the long hole lifting height is increased, and tunnel overexcavation is controlled; outer-insertion short holes are adopted, and tunnel bottom under-excavation is controlled. Full-cross-section excavation is adopted, a cross section is subjected to one-time excavation forming, the repetitive operation time of working procedures is shortened, construction organization and management are facilitated, the labor condition is improved, and the work efficiency is high.