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[Objective] The aims of this study are to explore the temporal and spatial variation characteristics of soil moisture in Robinia pseudoacacia plantation and Quercus liaotungensis natural secondary forest in the loess hilly region and its response mechanism to rainfall events, and reveal the soil moisture characteristics and differences between two typical forest communities in this region, so as to provide scientific basis for the construction and management of forest vegetation in this area. [Methods] The mature forest communities of two typical tree species in the loess hilly region were taken as the objects. By monitoring the soil moisture content in the range of 0-300 cm in the experimental site for 10 years (2009-2019), combining with the change of precipitation in the same period, the temporal and spatial variation pattern of soil moisture in these two typical forest communities and its response to rainfall were analyzed. [Results] (1) At the end of the growing season in 2019, the average soil water contents of the two forest communities at 0-100 cm and 0- 300 cm were higher than the average values in 2009, and there was a significant difference between the two communities in the range of 0-100 cm, and the average soil water content of Quercus liaotungensis forest in 2019 was significantly higher than that of Robinia pseudoacacia forest. The average value in the range of 0-300 em was significantly different only in Quercus liaotungensis forest. (2) The spatial distribution of soil moisture in the two forest communities fluctuated obviously in the 0-200 cm soil layer. The temporal and spatial stability of soil moisture in Quercus liaotungensis forest in 200-300 cm soil layer was higher than that in Robinia pseudoacacia forest. (3) The monthly dynamic trend of soil moisture in the two forest communities was basically the same, which was roughly synchronized with the monthly dynamic trend of rainfall, but the soil moisture in Quercus liaotungensis forest was always higher than that in Robinia pseudoacacia forest. (4) On the interannual scale, the soil water content of the two forest communities at the end of the growing season was significantly positively correlated with the rainfall in that year. With the increase of rainfall, the Quercus liaotungensis forest offset the evapotranspiration water consumption and supplemented the soil water more than the Robinia pseudoacacia forest. The increment of soil water storage in the two forest communities showed a very significant exponential growth relationship with the secondary rainfall. The supplementary effect of soil moisture in Robinia pseudoacacia forest was greater than that in Quercus liaotungensis forest when the secondary rainfall was small (less than 22 mm), but it was more beneficial to supplement soil moisture in Quercus liaotungensis forest when the secondary rainfall was large (more than 22 mm). [Conclusion] There are significant differences in temporal and spatial variation characteristics of soil moisture between two forest communities, especially in deep soil. Compared with Robinia pseudoacacia forest, the response of soil moisture in Quercus liaotungensis forest to interannual rainfall is more obvious. The difference of soil water storage in two forest communities to rainfall event depends on the amount of rainfall. [ABSTRACT FROM AUTHOR]