The invention discloses a fast measuring device for the inner diameter of building pipe fittings, which comprises two fixed cylinders whose middle parts are hinged to each other, a limit cylinder is installed in the fixed cylinder, and mutually cooperating limit blocks are fixed on the outer walls of the two limit cylinders , a steel ball is rotated at the bottom of the limiting cylinder, and the steel ball partially protrudes from the bottom of the fixed cylinder, a spring is installed in the fixed cylinder, the moving block is connected with the fixed seat through a scale, and the outer wall of the moving block is set There are a plurality of protrusions, and the inner wall of the fixed cylinder is provided with a plurality of grooves matching with the protrusions, and a ball is rotatably arranged on the upper surface of the fixed base. When the axis of the fixed cylinder coincides with the straight line where the diameter of the pipe is located, read the data on the scale, which is the displacement of the moving block in a straight line. Through calculation, the distance between the bottom end of the steel ball and the top end of the ball minus the straight line distance The linear displacement of the moving block can give the size of the inner diameter of the pipe.