Acetylene reduction was employed to study the nitrogenase activity (NA) of three types of biological soil crusts (algal, moss and lichen crusts) and the responses of NA to precipitation (1 , 3 and 5 mm), light intensity (0, 400 and 1 000 μmol/(m²·s)) and air temperature (13, 24 and 28 ℃) were also investigated under the laboratory condition. Results showed that NA differed significantly among crusts, with highest value occurring for algal crusts (16.6 mmol/(m²·h)), followed by lichen (6.9 mmol/(m²·h)) and moss crusts(2.6 mmol/(m²·h)). Precipitation and temperature significantly affected the NA, while light intensity had no effect on NA. The 3 and 5 mm precipitations significantly activated NA compared with 1 mm precipitation. NA was profoundly enhanced under the air temperature of 24 ℃ compared with 13 ℃. Therefore, firstly, this study indicated that biological soil crusts have the capability of nitrogen fixation, especially the algal crusts, which contributed profoundly to the soil nitrogen input in the desert ecosystems; secondly, the precipitation and temperature are the regulating factors for NA.