Abstract: Subgrade freeze-thawing injury is an important problem that has puzzled many engineers in the seasonal frost regions of northeast China such as the cit of Chang-chun. In winter. weather in Chang-chun is very frigidity. Water in subgrade is moving and assembling in some positions under gradient of ground temperature So, subgrade is frost heave. In spring, subgrade and ice lenses which assembled in some positions begin thawing. The subgrade soil is in saturation or over saturation states. The bearing capacity is very low. Under the action of traffic vehicles, bulges, elastic springs, fractures and soil boiling can appear on pavement, which give very bad injuries to communication, transportation, and economic construction. It is urgent and necessary to research every influencing factor that affects frost heave. Pores as one of important symbols of soil structure are main entryways of water movement in soil. They decide sensibility of frost heave of the soil. In the paper, the WD-5 SEM is used to collect microstructure photos of soils from three different locations nearby Chang-chun. The photos are further used to make quantitative analysis to microstructure of pores in soil. An image quantitative analysis system is used. The quantitative analysis results in the finding that microscopic pore diameters of three samples are mainly about 5 m. This pore size makes soils have property of frost heave. There aren't noticeable difference of microcosmic pores between vertical and horizontal sections from a same sample. There are radial or reticular structure of pores, and shapes of oblate or equiaxial. The analysis further leads to the finding that when pore shape fractal dimensions are higher, there are many big pores, and the pore distribution is simple. The research site is suitable for water movement in the subgrade, therefoe, the frost heave is strong.