The invention provides a double-side-wall composite cross method construction method for a large-span tunnel under the complex geological condition. The double-side-wall composite cross method construction method is used for solving the technical problems that during construction of a soft rock large-span tunnel, arch crown settlement is not easy to control and a support structure is prone to deforming, optimizing structural force, improving construction convenience and determining construction safety. According to the double-side-wall composite cross method construction method, pilot tunnelson the two sides are constructed firstly, and primary support side wall foundations are constructed in the pilot tunnels to serve as a bearing foundation of an upper primary support system, so that the problems that during construction of the soft rock large-span tunnel, arch crown settlement is not easy to control and the support structure is prone to deforming are solved; bases only serve as thebearing foundation of primary support, under the premise that the stability of the arch wall primary support system is guaranteed, an arch wall secondary lining can still be constructed through an integral formwork trolley, the technique is simple and convenient, the integrity of the secondary lining is guaranteed, the waterproof effect is good, and the construction quality is controllable; and the pilot tunnels on the two sides adopt the excavated section of an arc-shaped arch crown, compared with conventional pilot tunnels constructed through a double-side-wall heading method, the structureforce is more excellent, construction is convenient, and the construction quality of the support system is easy to ensure.