The present invention relates to a method for preparing polyethylene through a continuous gas phase method. The method comprises: S, carrying out nitrogen replacement and ethylene replacement, wherein the reaction pressure is controlled at less than or equal to 3.0 MPa, and the alpha-olefin partial pressure is 10-99% of the ethylene partial pressure; S2, introducing a metallocene catalyst; S3, controlling the reaction temperature and the reaction pressure; S4, detecting the hydrogen content in the circulation component, performing the operation of S5 when the hydrogen in the circulation gas is detected within 0.5 h after the catalyst is added to the reaction system, and re-performing the operation of S2 when the hydrogen in the circulation gas is not detected within 0.5 h after the catalyst is added to the reaction system; and S5, maintaining the process parameters of S3 to prepare the polyethylene product. With the method of the present invention, the reaction process can be accurately predicted, and the catalyst adding amount can be timely adjusted, such that the driving safety can be ensured, and the polymerization accident caused by the excessive initiation can be prevented, or the waste of a lot of nitrogen and ethylene due to the un-initiating of the catalyst can be prevented.