The invention relates to a gas phase polyethylene catalyst and a preparation method thereof. In the catalyst, anhydrous magnesium chloride and porous silica gel serve as a composite carrier, titanium-containing transition metal loaded on the composite carrier serves as an active component, an electron donor compound is added in the preparation process to serve as a modifier, and the electron donoris a silane compound with asymmetry. The problems that the common catalyst has low cavity, poor copolymerization ability and high product fine component content are solved, different silane electrondonor compounds are added in the preparation of the catalyst, polyethylene resin with different melt indexes is produced, in particular the gas phase polyethylene catalyst can be used for producing anethylene/1-hexene copolymerization new product with high additional value, so that the performance of the product is improved, the processibility of the resin is improved, and the application field of the product is widened.