The invention discloses a method for quickly building ethylene polymerization/copolymerization reaction in a gas phase fluidized bed reactor. The method is characterized by comprising the following steps: after a proper seed bed is filled, feeding nitrogen to replace trace water oxygen in a reaction system, switching into ethylene, hydrogen and copolymerization monomers for component adjustment, then initiating polymerization reaction by adopting an organic chromium family S-2 type catalyst, switching into a conventional chromium family catalyst, stopping adding the organic chromium family S-2type catalyst at the same time, and carrying out catalytic ethylene polymerization/copolymerization by adopting the conventional chromium family catalyst to produce a corresponding polymer. The invention provides a method for quickly building polymerization reaction in an ethylene polymerization/copolymerization gas phase fluidized bed reactor, and in the method, change of polymerization reactionparameters is not needed, the seed bed does not need to be switched, and a small quantity of raw materials are used and the energy consumption is low during replacement of the reaction system, so that production accidents such as sheeting of the reaction system, big static electricity of a polymerization device, high content of polymer fine powder, equipment shutdown and the like are avoided, andthe production period is shortened.