The purpose of this study was to determine the mineral-associated organic carbon (MOC) and its relationship to clay minerals under different temperatures and precipitation. We selected three typical grassland transects in China: Titanium Plate (TP), Mongolian Plate (MP), and Loess Plate (LP) with natural temperature gradients. Along the transect, there is a gradient in the precipitation between the various types of grasslands. The surface soil (0–10 cm) was sampled to determine the MOC. Clay minerals were charactered by X-ray diffraction (XRD). According to the findings, the MOC content increased with decreasing temperature (5.41–14.89 g/kg). MOC had a positive correlation (r = 0.67) with the amount of clay mineral content. In the large-scale study of transects, precipitation masks the effect of temperature change on the MOC to a certain extent. It indirectly affected the MOC content by affecting the mixed-layer illite/smectite (I/Sme) content, and this effect was strongest at the lowest temperature TP (r = −0.73). Except for precipitation, CaO in the soil can affect soil organic carbon (SOC) stability by influencing the pH and I/Sme. The amount of bacteria increased as a result of I/Sme, and the influence of bacteria on the MOC was surpassed only by the soil pH. Climate and clay mineral composition characteristics affected the MOC to a certain extent. Among them, the effect of precipitation change on the MOC is higher than temperature, but it has little effect on soil with a higher weathering degree and CaO content.