Understanding the temporal and spatial variation of soil moisture of forest and studying the effects of vegetation transpiration, forest floor evapotranspiration and meteorological factors on soil moisture are important to vegetation restoration, integrated forest-water management and the improvement of the eco-hydrological functions of forest/vegetation in the arid regions. A stand plot of Larix principis-rupprechtii plantation was established in the semiarid Diediegou small watershed on the north side of Liupan Mountains, Ningxia. Meteorological conditions, forest transpiration, forest floor evapotranspiration and soil moisture during July to October in 2013 were simultaneously monitored by automatic meteorological station, thermal diffusion probes, micro-lysimeters and time domain reflectometry. The soil moisture was monitored in the layers of 0-20, 20-40, 40-60 and 60-80 cm, to explored the main factors affecting the soil moisture. The results showed that:(1) Affected by the random rainfall events, the soil moisture showed corresponding pulse variation. On the whole, the mean soil moisture (32.69%) of the surface layer (0-20 cm) was relatively lower than that of the layers followed (about 40.00%). The variation range of soil moisture decreased gradually with the increasing of soil depth and rainfall amount. (2) The main meteorological factors influencing soil moisture were temperature, saturation vapor pressure deficit and atmospheric pressure. The forest transpiration and forest floor evapotranspiration had a significant correlation with the soil moisture of the main root zone (0-60 cm) during the whole study period. (3) The correlation coefficients between soil moisture and all the influencing factors decreased with the increasing of soil depth, which were significant in the main root zone, but no longer significant in the soil layers below (60-80 cm). In summary, the forest soil moisture was affected by both the precipitation input and evapotranspiration output. The temporal variation of soil moisture in each soil layer was similar. However, the soil moisture of surface layer was low and with a larger variation range. The sensitivity and amplitude of soil moisture to the influencing factors decreased gradually with the increasing of soil depth.