Grouting is frequently used to prevent water-inrush disasters in tunnels and underground engineering. However, complex groundwater environment can significantly influence the grouting effect, especially the antiwashout performance. To accurately evaluate the antiwashout performance of grouting materials, a new method is proposed to design specific measurement devices and measurement procedures. This new method allows direct simulation of the injection of grout into water with different flow velocities. Thus, the antiwashout performance of grout under the scouring and diluting action of flowing water can be evaluated quantitatively. The grout retention ratio (GRR) is defined to quantitatively determine the antiwashout performance of grout. Taking widely used cement–sodium silicate (C-S) grout and polyurethane grout as test samples, a series of tests with different mixing ratios and water flow velocity conditions was carried out. The feasibility of the new method was evaluated in these tests and the influences of the grout mixing ratio and water flow velocity on the GRR were discussed. By analyzing the experimental data, a polynomial regression model for C-S grout was derived to provide guidance for optimizing its antiwashout performance. Compared with C-S grout, polyurethane grout has much better antiwashout performance at higher water flow velocity, indicating its advantage in controlling the flow rate of water and preventing water-inrush disasters.