The invention discloses a ventilation smoke-dissipation system for an underground cavern during a construction period and a mounting method of the ventilation smoke-dissipation system. The ventilation smoke-dissipation system for the underground cavern during the construction period comprises a supply fan, an exhaust fan and a fan drum, wherein one end of the supply fan is arranged outside a cavern opening; the other end of the supply fan is connected with the fan drum to be used for supplying air into the cavern; one end of the exhaust fan is arranged in the cavern; the other end of the exhaust fan is connected to the outside of the cavern; the ventilation smoke-dissipation system further comprises a fan support used for supporting and fixing the supply fan and the exhaust fan; and the fan support is detachably connected with the supply fan and the exhaust fan. By the adoption of the ventilation smoke-dissipation system disclosed by the invention, wind resistance happening due to the fact that the fans and the fan drum are mounted irregularly is reduced, and the ventilation smoke-dissipation effects of all cavern working faces are guaranteed.