Microorganism plays an essential role in the decomposition of litter and senesced fine root, as a consequence of which more and more attentions have been paid to the microbial biomass dynamics in litter and soil in the past two decades. Moreover, increasingly studies have also demonstrated that lots of litter mass lost in cold winter in the seasonal freeze-thaw area. However, little information has been available on the dynamics of microbial biomass during litter decomposition in wintertime, far less for fine roots decomposition. In order to character the dynamics of microbial biomass during fine roots decomposition in a alpine forest over the growing season and freeze-thaw season, microbial biomass carbon (MBC), nitrogen (MBN) and phosphorus (MBP) were measured using the litterbag method from November 11, 2009 to November 11, 2010. The fine roots of Picea asperata,Abies faxoniana and Betula albosinensi were selected which are three representative tree species in the alpine and subalpine forest of Western China. Microbial biomass C(MBC) displayed a sharp decline tendency before the soil frozen as soil temperature decreased, and significantly increased utile to soil thawing, but continually declined as temperature increased after the middle stage of soil thawing, and then a increased tendency was observed. However, the changed time of MBC was different in the fine root decomposition of different species. The peak values of MBC during the decomposition of spruce and fir fine roots were observed in the late stage of growing season, but which of birch was detected in the early stage of soil frozen. MBN displayed the similar dynamics pattern during the fine roots decomposition for all three species, which rapidly decreased until soil completely frozen, and showed little changes during the whole freeze-thaw season, but obviously increased to a peak value in the later stage of growing season. In addition, MBP during the decomposition of spruce and fir fine roots were increased as the decomposition proceeded, although which of birch displayed its maximum value in the later stage of soil thawing, another peak value in the middle stage of growing season, and then obviously declined in the later stage of growing season. The results indicated that large quantities microorganisms could be available even in winter, which could do great contributions to the litter decomposition. Nevertheless, the microbial biomass could be significantly affected by fine root quality, temperature and other climate conditions.