The present invention relates to an elastic terpolymer that can acquire good mechanical properties and elasticity (flexibility) at the same time, and a preparation method thereof. The elastic terpolymer, which is a copolymer comprising 40 to 70 wt % of ethylene, 20 to 50 wt % of a C3-C20 alpha-olefin, and 2 to 20 wt % of a diene as obtained in the presence of a catalyst comprising a vanadium compound, satisfies that: (i) the weight average molecular weight measured by GPC is 100,000 to 500,000; and (ii) in the monomer distribution of the copolymer determined by 13C NMR analysis, K given by a defined Equation 1 is 0.01 or less.