Exploring how soil dissolved nitrogen pool fractions，nitrogen transformation processes and key enzyme activities of nitrogen metabolism respond to freezing-thawing cycle during non-growing season is important for supporting soil fertility sustainment and farmland nutriment management.With the farmland black soil as the research object，a laboratory simulation experiment was conducted to analyze the changes of soil dissolved nitrogen pool fractions [dissolved inorganic nitrogen (NH4+-N + NO^3--N，DIN)，dissolved organic nitrogen (DON)，dissolved total nitrogen (DTN)]，net nitrogen mineralization rate (NNMR)，microbial biomass nitrogen (MBN) and key enzyme activities of nitrogen metabolism under the freezing-thawing cycle for clarifying mechanisms of the effects of freezing-thawing cycle on soil dissolved nitrogen transformation.Freezing-thawing cycle had significant influence on dissolved nitrogen pool fractions，nitrogen transformation processes，MBN and key enzyme activities of nitrogen metabolism in the farmland black soil (P<0.05). The contents of NH4+-N、NO^3--N、DIN and NNMR decreased significantly in response to increasing cycles of freeze-thaw，but the contents of DON、DTN and MBN first increased then decreased，reaching the highest values at cycle number 6 with increment of 4.65%，5.43% and 18.38%，compared with the control check，respectively.With the increasing of freezing-thawing cycle，soil protease activity significantly increased，and nitrate reductase activity first increased then decreased，reaching the highest values at cycle number 6 with 1.19 NO₂--N μg/(g·24 h).Therefore，it is preliminarily concluded that the reasonable freezing-thawing cycle number (1～6) could significantly promote nitrogen transformation of farmland black soil during non-growing season，and was beneficial to the accumulation of soil available nitrogen.