SEM and direct shear tests were conducted to study the influence of freeze-thaw action on the microstructure and strength of xi'an Q3 undisturbed loess under the closed system. The experimental results showed that the microstructure of loess changes significantly during the freeze-thaw process, with the number of large particles aggregate decreasing, the proportion of small particle size increasing, and the ratio of pore increasing. Based on the damage mechanics theory, the freezing-thawing damage degree versus freezing-thawing times was established. Analysis indicated that the freezing-thawing damage degree exponentially increased with the increase of freezing-thawing times, representing that the structural strength of loess can be destroyed by freezing-thawing. However, the structural strength eventually reached a residual strength value after repeated freezing and thawing. Freezing-thawing made the surface structure of soil destroyed, and had a bigger influence with the increace of water content. The cohesion exponentially decreased with freezing and thawing times increasing, and had a smaller attenuation amplitude and rate with the increase of water content. With the increase of water content, the cohesion linearly decreased, and had an approximately same variation after freezing and thawing. The internal friction angle had no obvious variation during the whole freezing-thawing process. The freezing-thawing damage coefficient of cohesion exponentially increased with the increase of freezing-thawing times, and also increased with water content increasing. A cohesion strength deterioration model was developed based on the laboratory test results, and the model can be used to well describe the strength deterioration regularity by the test verification.