Abstract: Kimberlite is a kind of melange with changeable chemical composition, mineral composition and structure and tends to be altered. Therefore, it is difficult to determine the emplacement age of kimberlite by dating kimberlite whole rock and all kinds of minerals, and the data obtained are very different. In this paper, the intrusive relationship between kimberlite zone and diabase in Poli of Mengyin was analyzed. Through dating the zircon U-Pb in diabase and studying the contact relationship between diabase and overlying limestone as well as the relationship between diamond sand reservoir and known diamond protoores, the emplacement age of diabase vein was determined to be the Late Yanshanian period of Mesozoic. Therefore, it is proved that the Poli kimberlite belt was formed in Late Yanshanian rather than in Caledonian period.