In order to understand how forest gaps affect soil nitrogen dynamics, we estimated the dynamics of several soil nitrogen forms (ammonium nitrogen, nitrate nitrogen, microbial biomass nitrogen and dissolved organic nitrogen) over the four crucial periods (early-freezing period, deep-freezing period, early-thawing period and late-thawing period) through setting up four gap treatments (large gap, medium gap, small gap and under canopy) in an alpine Abies faxoniana forest of western Sichuan in the winter of 2012-2013. The results showed that, the contents of soil ammonium nitrogen and nitrate nitrogen were significantly higher in the late-thawing period than in the three other crucial periods; microbial biomass nitrogen was lowest in the early-thawing period, and soil dissolved organic nitrogen content was the minimum in the deep-freezing period, while both of which were the highest in the early-freezing period; soil nitrate content was high, accounting for 67.26%-83.59% of total soil mineral nitrogen. Forest gaps would change soil microenvironment and then change nitrogen components in winter. The size of forest gaps had a significant positive correlation with the dissolved organic nitrogen content; soil temperature was significantly positively correlated with ammonium, nitrate and dissolved organic nitrogen contents; while the soil freezing depth was significantly negatively correlated with soil nitrate and dissolved organic nitrogen contents. More mineral nitrogen and dissolved organic nitrogen were in the small gap and under canopy than in the large and medium gaps through seasonal freeze-thaw transition, thus creating a good growing condition for vegetation and soil microbes in growing season.