The invention relates to a propylene dimerization catalyst, its preparation method and application. It comprises potassium carbonate carrier and active component sodium and/or potassium whose contentis 1-10% by using total weight of catalyst as reference, which is characterized by that the more probable first particle grain size of the described potassium carbonate carrier is 0.5-1.5 micron, itsmore probable secondary particle grain size is 100-700 micron, total pore volume is greater than or equal to the power 5 A. Said catalyst not only is high in activity, good in selectivity and long inlife, but also possesses excellent comprehensive property and good controllability.