The invention discloses a water-blocking mould at a doorway of a ground heating toilet and a method for blocking water by using a water-blocking mould, belonging to tools and methods for building construction. The mould is in a cylindrical structure which has a rectangular section and penetrates through up and down; and a plurality of notches corresponding to ground heating tubes are respectivelyarranged at the bottoms of the front cylindrical wall and the rear cylindrical wall of the cylindrical structure. The method comprises the following steps of: when the concrete ground is cast, placing the mould at the doorway of the toilet and then casting concrete; after casting, taking off the mould and forming an arched notch at the mould part; and carrying out one waterproof operation at the arched notch and casting the concrete after the waterproof layer at the arched notch is formed, wherein the waterproof layer at the arched notch forms a water-blocking route, and during water closed experiment, the water in the toilet is blocked from entering a living room. The mould has a simple structure, is scientific in design, is placed at an outlet of the toilet and constructed according to the steps. According to the method, by closely sealing tiny clearances at the contacted part of the ground heating tubes and the concrete, the defect that a water source of the toilet enters the living room from the uncompacted leaks at the contacted parts of the ground heating pipes and the concrete is effectively overcome; and the method is simple and practical.