The action of freeze-thaw is one of the important influence factors which can cause the changes of soil physicochemical and biological characteristics. During the freeze-thaw period the variations of active organic matters in cropland black soil were studied through the in-situ monitoring and determining methods. The surface soil experienced a series of repeated freeze-thaw actions during the alternate from autumn to winter and from winter to spring. The soil active organic matter was in the trend of fluctuation increase from 8.68 mg kg-1to 9.81 mg kg-1 during the seasonal freeze-thaw period. The soil water soluble organic matter was also in a gradually increasing trend, which could increase 33.95% for the surface layer in early April of the next year when compared with that of January. The soil microorganism carbon content increased firstly and then decreased during the seasonal freeze-thaw period, which fluctuated at 912.51-1137.91 mg kg-1. The research showed that the freeze-thaw action could improve the active organic matter and strengthen the adaptability and activity of microorganism, which could promote the mineralization of organic matter.