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The spatial distribution characteristics and temporal variation trends of soil moisture significantly affect terrestrial water, energy, and carbon cycles at various scales. Satellite remote sensing is highly expected to provide such valuable information. Before applying the remotely sensed soil moisture products, a thorough validation must be conducted to insure product quality. In this paper, we evaluate the soil moisture products retrieved from the European Space Agency Soil Moisture and Ocean Salinity (SMOS) mission and the Advanced Microwave Scanning Radiometer 2 (AMSR2) on board the Global Change Observation Mission - Water (GCOM-W) over Heihe river basin in China, respectively. The land cover in Heihe river basin changes from desert to grass, agriculture field, and then mountain forest, which makes the basin an obvious spatial variation in soil moisture field and valuable to check the reliability and stability of two soil moisture products. We calculate the diurnal relative difference (DRD) of monthly averaged soil moisture between day and night observation of each products, and comparing them with that calculated from corresponding Global Land Data Assimilation System (GLDAS) simulations. The comparison results indicate that the SMOS soil moisture products are much unstable than AMSR2 retrievals. The DRD of SMOS is 20 times larger than that of AMSR2 and 100 times larger than that of GLDAS. We speculate that the radio frequency interference effects on SMOS observation may contribute to this unstable performance. Moreover, the retrievals from multi-angle observations in SMOS algorithm is also a potential source causing this systemic bias.