A flush control valve core assembly is provided including a valve core body, a back spring accommodated in the valve core body, a valve piece and a valve shaft. The valve piece includes a discharge hole. A portion of the back spring extends as a pin structure, which is able to slidably move in the discharge hole. The pin structure is formed by a free end of the back spring horizontally bending towards the center, and then bending downwards vertically. The back spring of the flush control valve core assembly has a smaller size, which is more reliable and more easily mounted.