Provided is a method of preparation of a spherical support for an olefin polymerization catalyst. Specifically, provided is a method of preparation of a support which can be used in preparation of an olefin polymerization catalyst, wherein, MgX(I) (X=halogen atom) is prepared by reacting a reaction initiator, nitrogen halide, and magnesium metal, and then magnesium metal and alcohol are reacted in the presence of the MgX, thereby resulting a smooth-surfaced spherical dialkoxy magnesium support having a uniform particle size distribution.