The present invention relates to a large-span tunnel excavation method for upper soft and lower hard compound strata. The present invention aims at providing a large-span tunnel excavation method for upper soft and lower hard compound strata so as to ensure large-span tunnel construction safety for the upper soft and lower hard compound strata, accelerate the construction progress and shorten the construction period. The technical scheme is that a tunnel is divided into upper and lower steps for excavation by the method, and the steps are as follows: a, determining a boundary of the upper soft and lower hard strata of an excavation surface according to geological prospecting data; b, constructing the upper step by using the center diaphragm method; c, constructing the lower step by using the method of reserving core soil; d, lagging the lower step with 2-3m headings at two sides; e, arranging earthwork cloth and an EVA waterproof board on a surface of a lower step inverted arch preliminary bracing, and pouring an inverted arch second lining; and f, dismounting a center diaphragm temporary bracing of the upper step and a temporary inverted arch of the upper step with segments, and pouring a pavement lining close to a preliminary bracing of the upper step and preliminary bracings of the headings at the two sides of the lower step. The large-span tunnel excavation method is applied to the technical field of tunnel construction.