Abstract: Nitrogen (N) is the most important element for the growth and yield of crops. Biological N fixation plays an essential role in N supply in organic agriculture without chemical fertilizer input. However, the effect of organic management years on biological N fixation activity is still unclear. This study compared the dynamics of N fixation rate and cumulative N fixation in soils under the organic management of cauliflower for 10 years, 14 years, 20 years and its conventional management for 20 years. And the factors influencing N fixation were explored using multivariate statistical analysis. The results showed that during the 7-day incubation, 10-year and 14-year organic managements reduced the N fixation rate compared with conventional management. However, the accumulative N fixation over the incubation showed no significant difference among all treatments. The N fixation rate and cumulative N fixation amount were significantly affected by organic management years and were the lowest in 14-year organic management. At the end of incubation, the soil N fixation rate under conventional management was near zero, whereas N fixation activity remained high under organic managements. Pearson correlation and redundancy analysis showed that the N fixation activity was primarily affected by the contents of NH₄⁺, NO₃⁻ and Olsen P. NO₃⁻ suppressed the N fixation, while Olsen P increased the N fixation activity.