Soil nematodes are one of the most important components in terrestrial ecosystems and the critical factor driving the belowground process. The grasslands of Northeast China have been subject to mowing for ages, which theoretically should have had substantial effects on the processes associated with soil nematodes. However, relevant studies have barely been conducted to date. This study examined variations in soil nematode abundance, biomass, diversity, and community structure, with respect to varying mowing frequencies. The results showed that a higher mowing frequency significantly reduced the abundance of soil nematodes, biomass, diversity, and community structure stability in the ecosystem, while intermediate mowing frequency enhanced these parameters to different extents. Our findings indicate that the changing patterns of the nematode indices with mowing frequency conform to the intermediate disturbance theory. This study provides a theoretical basis for formulating grassland-related management measures and maintaining the stability of grassland ecosystems.