Part of soil under the hard shoulder should be improved to reach the design standards of subbase in Hangzhou-Ningbo freeway widening project. Injection method is firstly put forward to improve the soil, and experimental study on injection process and effect is introduced in this paper. Researches about location of injecting hole, injection material and method selecting,injection controlling parameters and construction techniques are studied. In the end,two groups of testing are done to validate the reinforced effect. It is shown that as the deflection value decreases, modules of resilience and shear wave speed increase remarkably after injection. The resilient modulus of injected soil reaches the design standards. The advantage of this method is that it causes little influence on traffic and 28 percents of total engineering cost can be saved.