Abstract: In order to obtain the relation of undisturbed loess’ pore size distribution and mechanical property, with undisturbed loess as the research object to carry out the testing of NMR observation and triaxial shear under repeated freeze-thaw action. The experimental results show that in a freeze-thaw environment loess’ strength and internal pore structure change, with the number of freeze-thaw increasing, soil’s cohesion decreases and tends to stable finally. And its internal friction angle is basically stable, slightly reduced, and the change is between 1°~2°. Repeated freeze-thaw action makes the fine particles of soil increase; In the aspect of pores, the change of small pores’ number is from decreasing to increasing, while the change of middle and large pores’ number is from increasing to decreasing. With the freezing temperature gradually decreasing, the cohesion of soil will increase, however，the internal friction angle of undisturbed soil is basically stable.