The invention relates to a construction method for a gradual-change large-span tunnel. The construction method includes the following steps that when a three-lane excavation process reaches a gradual-change large-span intersection, a large-span section is excavated with three-lane spans as guide holes, and arch frames are closely distributed and reinforced at the intersection; when the three-laneguide holes of the large-span section are excavated, the guide holes slowly reach a designed height of the large-span section, the guide holes of the large-span section are sequentially excavated to alarge-span E section according to a step method, upper and lower steps of the guide holes are sequentially constructed, and concrete is sprayed; after the construction of the large-span E section iscompleted, transverse expanded excavation is carried out to a designed profile of the large-span E section, and an overall initial support is constructed; reverse expanded excavation is conducted on the large-span section from the large-span E section to a design profile of a large-span A section according to a three-step method, large-span secondary lining is conducted after the large-span initial support is completed, and reverse excavation is conducted from the large-span section to two lanes of a bifurcate section. According to the method, a construction method of the large-span tunnel issimplified, and the construction efficiency is improved under the premise of ensuring the construction safety.