The invention discloses an internal mold and a method for manufacturing concrete pipe fitting with the internal mold. The internal mold comprises a body. A hollow cylindrical body provided with a containing cavity is arranged at the center of the body. The body comprises a clamping portion and a fixing portion. The clamping portion and the fixing portion can be clamped to each other or separated from each other. When the clamping portion and the fixing portion are clamped to each other, the body forms a closed ring. The fixing portion has elastic deformation to bend towards the axis of the fixing portion. When the clamping portion is separated from the fixing portion, the fixing portion is bent towards the axis of the fixing portion. A transmission mechanism is arranged in the containing cavity. The transmission mechanism is in transmission connection with the clamping portion and can drive the clamping portion to conduct reciprocating motion between the clamping position and the position inside the containing cavity in the radial direction of the body, so that the clamping portion and the fixing portion are clamped to each other or separated from each other. The internal mold and the method for manufacturing the concrete pipe fitting with the internal mold can solve the problems that in the production and manufacturing process of the concrete pipe fitting, an internal mold is prone to damage when taken out, and the production cost is increased.