As an important component of the earth ecosystem, soil moisture monitoring is of great significance in the fields of crop growth monitoring, crop yield estimation, variable irrigation, and other related applications. In order to mitigate or eliminate the impacts of sparse vegetation covers in farmland areas, this study combines multi-source remote sensing data from Sentinel-1 radar and Sentinel-2 optical satellites to quantitatively retrieve soil moisture content. Firstly, a traditional Oh model was applied to estimate soil moisture content after removing vegetation influence by a water cloud model. Secondly, support vector regression (SVR) and generalized regression neural network (GRNN) models were used to establish the relationships between various remote sensing features and real soil moisture. Finally, a regression convolutional neural network (CNNR) model is constructed to extract deep-level features of remote sensing data to increase soil moisture retrieval accuracy. In addition, polarimetric decomposition features for real Sentinel-1 PolSAR data are also included in the construction of inversion models. Based on the established soil moisture retrieval models, this study analyzes the influence of each input feature on the inversion accuracy in detail. The experimental results show that the optimal combination of R² and root mean square error (RMSE) for SVR is 0.7619 and 0.0257 cm³/cm³, respectively. The optimal combination of R² and RMSE for GRNN is 0.7098 and 0.0264 cm³/cm³, respectively. Especially, the CNNR model with optimal feature combination can generate inversion results with the highest accuracy, whose R² and RMSE reach up to 0.8947 and 0.0208 cm³/cm³, respectively. Compared to other methods, the proposed algorithm improves the accuracy of soil moisture retrieval from synthetic aperture radar (SAR) and optical data. Furthermore, after adding polarization decomposition features, the R² of CNNR is raised by 0.1524 and the RMSE of CNNR decreased by 0.0019 cm³/cm³ on average, which means that the addition of polarimetric decomposition features effectively improves the accuracy of soil moisture retrieval results.

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