The invention relates to a mechanical automatic shutoff gate valve, which comprises a turbine, transmission reducer and an automatic shutoff device. The turbine comprises a volute shell, an impeller is arranged in the volute shell, the middle part of the impeller is provided with an impeller shaft, the two ends of the volute shell are respectively provided with an oblique port, the outer sides of the two ends of het volute shell are respectively provided with a diversion net, and the outer sides of the diversion net are respectively provided with an end cover; the transmission reducer comprises a shell, a transmission wheel is arranged in the shell and is engaged with a main wheel, wherein the radius of the transmission wheel is smaller than the radius of the main wheel, the transmission wheel is provided with a transmission wheel shaft, the transmission wheel shaft is fixedly connected with the impeller shaft, the main wheel is provided with a main wheel shaft, and the main wheel shaft is provided with scales; the automatic shutoff device comprises a brake wheel, the brake wheel is provided with a groove, the upper part of the outer side of the brake wheel is provided with a brake lever, the end of the brake lever is provided with an elastic sheet, and the main wheel shaft is fixedly connected with the brake wheel. The invention has a simple structure, and can accurately control the total amount of the effluent completely through the mechanical device, realize the automatic shutoff, and realize the stepless regulation of the total amount of the effluent within the range of the maximum amount of the effluent.