The invention discloses an on-line cleaning method for a low density polyethylene production gas phase fluidized bed reaction system. A part of gas is drawn from a gas phase fluidized bed, is used as a circulation air flow and is circulated, in on-line cleaning, an inert condensable component is added into the circulation air flow subjected to compression and condensation, the inert condensable component has a solubility parameter of 6.5-9cal<1/2>. cm<3/2>, through compression and condensation, the circulation air flow forms a gas-liquid two phase mixture, and the mixture is returned into the gas phase fluidized bed so that a circulation loop is formed. Condensation depth and on-line cleaning time are adjusted and controlled and the reaction system is subjected to on-line effective cleaning so that device running time is prolonged and the problem that a distribution plate and a heat exchanger are blocked, system differential pressure is improved and a heat dissipation capability is reduced in long-time cycle operation of the low density polyethylene production gas phase fluidized bed reaction system is solved.