The invention discloses a method for excavating and supporting a large-span level-crossing fork of an underground chamber. The method comprises the following steps: firstly, excavating in the middle of a main hole and carrying out superficial layer support, and then excavating superficial layer supports on two sides of the main hole and two sides of a branch hole; deeply anchoring the ceiling part of an arch of the fork by an anchor rope; finally, excavating the surrounding part of a rock pillar from one side of the rock pillar to the other side of the rock pillar; carrying out superficial layer support when excavating; inserting encrypted opposite-penetrating anchor rod or anchor rope to reinforce inside superficial layer support; hanging a large-diameter reinforcing mesh at the weak part of a pier head structure of the rock pillar; connecting the reinforcing mesh and the opposite-penetrating anchor rod into an integral body in a welding manner, and re-spraying concrete to seal a large reinforcing mesh. The method is simple and convenient, applicable, and strong in maneuverability; the safety during the construction period and the engineering safety are facilitated.