[in Chinese with English abstract: ] Collema tenax is an important component of biological soil crusts in arid and semiarid ecosystems where its nitrogen fixation is important in ecosystem N cycling. This lichen occurs in surface soil with sharp temperature and water fluctuations, but its N fixation response to freezing-thawing in northern China is yet unknown. Differences in N-fixing activity of C. tenax among four freezing-thawing events under two thallus water conditions (wet-freezing: thallus water content > 200% dry weight; dry-freezing: thallus water content < 20% dry weight) were measured using acetylene reduction assay. N-fixing activity ranged between 2,371.0－ 8,701.8nmol C2H4/m2·h and followed the decreasing order of control > dry-freezing > wet-freezing. N-fixing activity (% of the control) of both dry-freezing and wet-freezing shows significant negative linear relationship with freezing-thawing times (R2 = 0.916 for dry-freezing and R2 = 0.965 for wet-freezing, all at p< 0.001). The absolute value of slope of wet-freezing (25.05) is much higher than that of dry-freezing (7.06). These results show a higher sensitivity of N fixation of C. tenax to freezing in wet than in dry condition. The lower sensitivity to low temperature in N-fixing activity for dry thallus of C. tenax is likely a physiological adaptation strategy to climate of northern China that characterized by prolonged drought and sharp temperature fluctuations. Key words: lichen, acetylene reduction assay, cryobiology, nitrogenase activity.