Soil moisture (SM) is a comprehensive variable of terrestrial ecosystems. However, the global SM trend in recent decades and the future is uncertain, and the main factors causing soil drying and wetting are not fully understood. This study using Extended Triple Collation, Mann-Kendall test, Theil-Sen estimate, Hurst exponent, Ensemble Empirical Mode Decomposition and pertinent methods found that 1) at the pixel scale, the long-term daily SM data from ERA-Interim was closest to the true SM compared with ESA CCI and GLDAS; 2) the global average SM in 1979–2017 decreased remarkably and the declining trend accelerated in 2001–2017; 3) soil drying was observed in seven major land covers; among which, urban area has the largest area proportion of soil drying, about 80%; 4) the strong persistence of SM trend indicates that the global future soil will continuously be dominated by a drying trend; 5) overall, 65.1% of the global soil drying trend was attributed to temperature rising, whereas 82% of the wetting trend was affected by the combined action of temperature, precipitation, and NDVI. Under global warming, the soil drying area expanded at a rate of 1% yr？1 in 1979–2017 and the global SM will keep diminishing in the next years, which may increase the risks of extreme heatwaves, water resources shortage, land degradation, and other eco-environmental problems.