The invention discloses a hanging mounting method of a long heavy stay cable under a space limited condition. The hanging mounting method comprises mounting of a first stay cable body on the center span side of a cable tower and a second stay cable body on the side span side of the cable tower and comprises the following steps of S10, tower end mounting of the first stay cable body; S20, beam end mounting of the first stay cable body; S30, tower end hard traction of the first stay cable body; S40, tower end mounting of the second stay cable body; S50, beam end mounting of the second stay cable body; and S60, synchronous different-place tensioning. A tower-first beam-second hanging manner is adopted in the stay cable bodies on the center span side and the side span side for hanging, and the problems that in the beam-tower end hanging scheme of the long heavy stay cable, during rear-tower-end hanging, traction force is too large, and safety is poor are solved. Meanwhile, beam end and tower end synchronous different-place tensioning is adopted, the problems that upper tower column space is small and narrow, and construction is difficult are solved, the phenomenon that when the stay cable bodies are tensioned each time, a set of tensioning extending rods are cut to be wasted is avoided, material waste is reduced, and the construction cost is reduced.