Based on the process of reinforcing soft soil using cement in laboratory，the porosity of test samples under different pressures conditions were determined through mercury intrusion porosimetry． The experiment was introduced and the law of porosity variation was summarized． The results show that when stress is between 0 and 0. 15--0. 2 qu ，porosity decreases along with the pressure，then porosity has increased in the wake of the pressure until the sample damaged． It can provide an experimental basis for exploring the failure mechanism of cement stabilized soil from the microscopic view and the study to establish the microcosmic constitutive model of cement stabilized soil．