Variance and coefficient of variation were used to analyze temporal variability of soil organic matter (SOM) content in the Nihegou watershed in the loess plateau from 1980 to 2015, and isolated the effects of factors on total variation. The results showed that SOM content of cultivated land showed an increasing trend since 1980, the variability increased first and then decreased. Building terraces, slope protection and increasing fertilizer application had great effects on the variation of SOM content, and the effect of increasing fertilizer application was slightly more than that of building terraces. Building terraces caused the loss of SOM in the initial stage due to soil disturbance. Although slope protection only played a role in the smaller area, it avoided soil disturbance thus had a better effect. The variation caused by all factors was a process of long-term accumulation, in which SOM did not increase endlessly. The higher the content of SOM, the less obvious the increase. SOM content of farmland increased from 9.52 g/kg in 1980 to 14.58 g/kg in 2015, which was improved from the 7th level to the 5th level in the 2nd national soil survey classification. The above results reflect the governance benefit of the watershed, and provide a basis for watershed management and management in the future.