The utility model relates to a flash tank for preparing reaction product hexene-1 of ethylene trimerization. The flash tank comprises a tank body, wherein the side wall of the tank body is provided with a feed hole; the outer wall of the tank body between the feed hole and the bottom of the tank body is provided with a heating device; the bottom of the tank body is provided with a polymer outlet; a baffle plate is arranged at the inner upper part of the tank body; the heating device is a heat medium jacket; and the side wall of the tank body is provided with a liquid phase outlet. By adopting the flash tank provided by the utility model, the problem of separation of a polymer in a flash process during the preparation of the product of reaction of hexene-1 by ethylene trimerization can be solved effectively, and even if the polymer is separated, pipes and systems for subsequent production can not be blocked by the polymer.