Soil inorganic carbon (SIC) is the main form of soil carbon in drylands and is susceptible to land use changes. However, a comprehensive study on the SIC dynamic after afforestation is still lacking. In this study, we collected 883 data from 57 published papers to investigate the effects of afforestation on SIC, and then used a random forest model to spatially estimate SIC dynamic following afforestation. The results showed that the SIC content significantly rose by 14.21% after afforestation. The SIC accumulation magnitude after cropland afforestation was less than that after grassland and sandland afforestation. Mean annual precipitation was the primary factor affecting the changes in SIC after cropland afforestation, and soil pH was key effect factor to SIC variations after grassland and sandland afforestation. The SIC variations following afforestation were directly or indirectly affected by climate factors, soil pH, total nitrogen, cation exchange capacity and response ratio of soil organic carbon (RR-SOC), indicating that pedogenic inorganic carbon is a key factor in SIC accumulation after afforestation. Spatial predictions of SIC dynamics following afforestation showed that SIC accumulation rate is comparable to that of SOC, indicating that SIC is equally important in soil carbon sequestration. In conclusion, our research results emphasized the significance of SIC, ignoring SIC could lead to a significant underestimation of the carbon sequestration capacity of the afforestation areas.