The invention creatively provides a construction method of a steel-concrete combined section of a long-span cable-stayed bridge. The construction method includes the steps that firstly, longitudinal prestressed tendons of concrete beams are installed, and fine-adjusted concrete beam sections are put in place; then a plurality of pouring holes are formed in steel beams, fine-adjusted steel box girders are put in place, and the steel beams and the concrete beams are in butt joint and locked temporarily; then steel geocell concrete of a bottom plate, a web, steel geocell concrete of an inclined web and steel geocell concrete of an outer web are poured; and then steel geocell concrete of a roof is poured. According to the construction method of the steel-concrete combined section of the long-span cable-stayed bridge, concrete is poured directly through a discharging hopper, the impact load on a support system during concrete conveying through a pump is avoided, shaking is relieved, the structural stress condition of the junction of the steel-concrete combined section is improved, strict vibration is not required during pouring, construction is convenient, the effect after curing is good, the concrete pouring quality is excellent, the safety risk in the construction process is lowered, and the construction progress is accelerated.