Abstract: The test apparatus for underwater karst grouting materials, which be used to evaluate the anti-washout properties of grout, is developed. Using this device, a series of anti-washout property tests on modified clay cement paste grout in underwater karst fillings are conducted. The retention ratio index of grout is utilized for the quantitative evaluation of the anti-washout properties of grout, and the tests on physical and mechanical properties of retained body are also carried out. The result shows that the anti-washout properties of modified clay cement paste grout are greatly affected by the water cement ratio of grout, karst fillings and velocity of water flow. The anti-washout properties of grout are relatively good when the velocity of karst water flow is no more than 0.8 m/s. With the increasing water cement ratio of grout or the decreasing water flow velocity, the retention ratio of grout tends to increase and can be significantly improved with the existence of karst filling materials. The physical and mechanical performance tests on the retained body indicate that the density is greatly influenced by the filling materials. When grouting under hydrodynamic environment, the downstream grout diffusion distance is greater than the upstream one. The viscosity of grout has a great effect on the diffusion distance, and the velocity of karst water flow can be in favour of the diffusion of grout along the water flow direction and restrain the diffusion along the reverse water flow direction. The 28-d compressive strength of retained body is deeply influenced by filling materials. The 28-d compressive strength under karst water condition is significantly less than that under freshwater condition, but it is still up to 3 MPa. The engineering application of prototype tests indicates that the modified clay cement paste grout can be used for treatment of karst areas under certain velocities of karst water. The formula for grout in underwater karst treatment derived by indoor tests is feasible and may provide theoretical guidance for underwater karst grouting of shield tunnels.