The quantitative dispensing machine mainly includes a compressed air source, a main compressed air pipeline, a pair of compressed air pipelines, a glue gun and a timing controller. A pressure regulating valve, a pressure gauge and a three-way valve are sequentially arranged in the main compressed air pipeline, one end of the main compressed air pipeline is connected with a compressed air source, and the other end is connected with a hose. An auxiliary compressed air pipeline connected with a compressed air source is sequentially provided with an air regulating valve and a branch pipe communicated with the atmosphere, so that the pressure in the branch pipe is negative pressure. In addition, the piston glue gun pushes the glue out of the needle with the precise control of the air pressure and timing controller, and at the same time, the negative pressure sucks the glue back, so that there will be no residual drops in the glue gun.