Abstract: In the process of fire prevention grouting, the critical speed was difficult to determine, which caused some problems in the elbow segment of thickening colloid conveying pipeline, such as severe vibration, easy rupture and so on. In order to ensure the safe and efficient operation of elbow segment in the grouting process, the mathematic model of thickening colloid conveying pipeline was established. ANSYS software was used to conduct numerical simulation for the different inlet speed conditions. The pressure, speed, deformation, stress and strain distributions were obtained under different conditions of inlet speed, and the grouting critical speed was determined. The simulation results coincided with the test measurement data, which provided a new theoretical foundation for fire prevention grouting technology.