A mathematical model for grouting into a fracture with flowing water is derived and validated by a model test and calculation. The results show that grout propagation shape is round in prophase of grouting; and it is elliptic in anaphase of grouting. Penetration radius is in inverse proportion to viscosity and is in direct proportion to the opening of fracture and grouting pressure. Water flowing is propitious to propagation downstream the current and restrained the propagation against the current. The influence of velocity will gradually reduce when it increase or decrease to a certain value. And the influence of velocity on the penetration length against the current is greater than on that of downstream the current. Velocity of flow has the most important effect on penetration radius; and the opening of fracture takes the second place. Grouting pressure and viscosity have less influence on penetration radius.