Diffusion law of dynamic grouting has directive significance to the grouting design and construction. Simulation tests of crack dynamic grouting with cement-silicate (C-S) slurry are carried out. The asymmetric ellipse (AE) law is revealed to describe the C-S slurry diffusion process under laboratory conditions. The concept of “speed ratio of slurry to water” is proposed to fit the AE parameters-time curve. How the speed ratio of slurry to water affects grouting diffusion range is analyzed. The transient equations describing slurry diffusion trace is established. The results show that: the slurry diffusion trace could be described with asymmetric ellipse which varies with time. The speed ratio of slurry to water is positively correlated with diffusion distance against flow direction and lateral diffusion distance. The speed ratio is negative correlated with diffusion distance along flow direction. Based on the results above, suggestions for grouting design are proposed.