The present invention discloses a gas-liquid-solid three-phase fluid mixing apparatus, which comprises a mixing cylinder, wherein the mixing cylinder comprises a liquid-phase injection section, a solid-liquid mixing section, a gas-liquid-solid three-phase mixing section, and a three-phase jet flow output section, the liquid-phase injection section, the solid-liquid mixing section, the gas-liquid-solid three-phase mixing section and the three-phase jet flow output section are sequentially communicated, the liquid-phase injection section is provided with a liquid-phase injection port, the solid-liquid mixing section is provided with a solid-phase injection port, the gas-liquid-solid three-phase mixing section is provided with a gas-phase injection port, the three-phase jet flow output section is provided with a three-phase jet flow output port, the liquid-phase injection port of the liquid-phase injection section is provided with a necking structure, the solid-phase injection port and the gas-phase injection port are annular injection ports, the included angle of the solid-phase injection port and the flowing direction of the fluid in the mixing cylinder is 45-60 DEG C, and the included angle of the gas-phase injection port and the flowing direction of the fluid in the mixing cylinder is 45-60 DEG C. According to the present invention, the internal structure of the existing mixing apparatus is changed and the gas phase mixing is arranged so as to fundamentally change the characteristic of the flow field, and with the solid particle stirring effect, the liquid phase and air mixing effect is strengthened so as to achieve the large flow spraying of the positive pressure type foam generator, improve the foam foaming performance, and increase the fire extinguishing efficiency.