Abstract: Flow pattern, time-varying behavior of rheological parameters, groutability, plastic strength and re-groutability of grouts are properties influencing the diffusion of grouts. Test result indicates that the flow pattern of grouts can be classified into three types, namely Newtonian fluid, Bingham fluid and Power law fluid. The time-varying behavior of grouts conforms to exponential function, and the flow pattern of cement-based grouts does not change during grouting. Grouting and back grouting tests are conducted with a simulation grouting device, verifying that the minimal groutable breadth of fissures are not 0.18mm or 0.20mm but changeable with different water-cement ratio, and the grouts can force a new way in the flow path during back grouting as long as the pressure of back grouting is larger than the cohesion of grouts previously injected into the fissures, rather than push them forward as a whole. In addition, the concept of plastic strength is proposed to predict groutability period of grouts. Based on these achievements, especially on time-varying behavior of viscosity of grouts, the author develops a grout diffusion model.