The invention relates to a catalyst for ethylene and alpha-olefin copolymerization and a preparation method thereof. The catalyst comprises a magnesium compound, a titanium compound, an electron donor I and an electron donor II. The electron donor I is an ester compound. The electron donor II is a siloxane compound. The preparation method of the catalyst includes that the magnesium compound is dissolved in a hydrocarbon solvent and an alcohol solvent, and reacts for 2-4 hours under the temperature of 70-90 DEG C; the electron donor I is added and reacts for 0.5-2 hours, and then the electron donor II is added and reacts for 0.5-2 hours; the titanium compound is added dropwisely under the room temperature, and reactions undergo 1-3 hours under the temperature of 70-90 DEG C; the solvents are eliminated; the hydrocarbon solvent is added; after the electron donor II reacts for 1-2 hours, the electron donor I is added and reacts for 1-2 hours; the titanium compound is added and reacts for 1-3 hours; and filtering, washing and drying are carried out to produce the catalyst. According to the catalyst, the catalyst activity for the ethylene and the long-chain alpha-olefin copolymerization is high, so the inserting rate of the long-chain alpha-olefin of produced copolymer is high.