This study was aimed to explore the main influencing factors and response functions of the soil organic matter (SOM) content of black locust (Robinia pseudoacacia) plantations on the Loess Plateau of northwest China, and to provide scientific basis for the studies on the carbon sequestration service of black locust plantations. In this paper, 136 papers published in domestic and foreign journals and academic dissertations in the period of 1994—2022 related on the topic of SOM content of black locust plantations in the Loess Plateau were collected, and the data of 330 plots were extracted, for analyzing the effects of main factors on the SOM content using the upper boundary line method and geo-detector method. The upper boundary line method analysis showed that the SOM content was nonlinearly influenced by many factors. It first increased and then decreased with rising altitude, mean annual air temperature and forest age, but first rapidly and then slowly decreased with rising soil layer depth, first gradually and then slowly increased with rising mean annual precipitation, and decreased linearly with rising slope gradient. The geo-detector analysis showed that the contribution rate of single influencing factors to the variation of SOM content was: altitude > mean annual precipitation > mean annual air temperature > soil layer depth > forest age > slope gradient. The explanatory force of the interaction of any two factors to the variation of SOM content was greater than that of single factor, and the interaction effect between altitude and soil layer depth was the largest one (as high as 94.2%). In summary, the SOM content of black locust plantations on the Loess Plateau was non-linearly influenced by many factors. The influences of regional environmental factors (altitude, mean annual precipitation, mean annual air temperature) were higher than those of local factors (soil layer depth, forest age, slope gradient). The interaction between main influencing factors should be considered as well.