In grassland, introduction of legume potentially can play a significant role in increasing soil carbon (C) and nitrogen (N) storage through increasing biomass C input. It has been reported that grass–legume ratio (GLR) is important in assessing benefits of grass–legume mixtures, however, little is known about how GLR affects soil C and N storage. As an effort to understand the effects and mechanisms of GLR on soil carbon and nitrogen storage, biological nitrogen fixation (BNF) by legumes, and changes in C and N storage in plants and soil were examined under swards with different densities of grasses and legumes over 4 years in a temperate steppe grassland. Results showed that total N storage in 0–40 and overall 0–150 cm soil depths varied as GLR changed, reaching peak values when the GLR was equivalent to 1:1. Increased relative legume abundance significantly enhanced soil C storage in both 0–40 and 0–150 cm soil depths, which was primarily attributed to an increase of aboveground biomass C. Soil organic C and total N varied with GLR changes, leading to stoichiometric shifts in soil C:N ratios, possibly influencing soil C storage through changing plant community structure and soil biochemistry. It is suggested that introducing legumes in steppe grasslands with a GLR of 1:1 could increase resources use efficiency and improve the sustainability of the environment.