The invention discloses a method of preparing an olefin polymer, which includes the following steps: carrying out a copolymerization reaction to an olefin monomer in a gas-phase fluidized bed reactor to obtain the olefin polymer, feeding the olefin polymer into a degassing bin to perform devolatilization, wherein the initial concentration of the olefin monomer in the gas-phase fluidized bed reactor is controlled so that the partial pressure of the olefin polymer in the degassing bin is lower than the saturated vapor pressure of the olefin polymer at the devolatilization temperature. The method can effectively prevent agglomeration of the olefin polymer granules, is saved in cost in after-treatment and is simple in processes. The polymer is low in density, is wide in distribution and is uniform in crystallization degree, and can be widely used in the fields of film materials.