Taking cropland soil in the Mollisol region of Northeast China as object, an indoor experiment was carried out to have the soil subjected to simulated freezing-thawing alternation for investigation of effects of the alternation on macro-pores in the soil profile using CT (Computerized tomography) thorough quantifying and analyzing variation of the number, average area, Feret diameter and roundness of macro-pores with soil depth (0~40 cm，40~80 cm，120~160 cm). Results show that the alternation did not change the characterization of macro pores in 0~40 cm but significantly reduced the average area and Feret diameter of pores in 40~80 cm by 46.72% and 17.32%, respectively (p<0.05). Pores, 1~2 mm and ≥5 mm in diameter were found most vulnerable to the alternation, decreasing by 9.58% and 42.19% (p<0.05), respectively. The findings confirmed the possible effect of argillic horizon stagnating water, and susceptibility of the pore structure in argillic horizon to the alternation, thus demonstrating that seasonal freezing-thawing alternation is the major cause of surface soil erosion and gully erosion of the Mollisol region through weakening erosion resistance of its argillic horizon and enhancing the effect of the argillic horizon stagnating water.