This thesis aims to improve our understanding of soil physical property retrieval by using a data assimilation (DA) system that includes a land surface model (LSM) acted as a system operator and a microwave emission model as a measurement operator. The thesis first focuses on soil physical properties for land surface modeling on the Tibetan Plateau (TP) (Chapter 2) due to the unrepresentative soil property dataset adopted in some LSMs there. For this purpose, we collected and compiled the soil physical property profiles on the different climate zones of the TP, and used them to examine various schemes for estimation of soil hydraulic properties (SHP) and thermal properties (STPs) and quantify the uncertainties in existing basic soil property datasets on the TP. In microwave L-band emission modeling, to account for the effect of topsoil structures and inhomogeneous moisture distribution in the soil volume on L-band radiation, we developed an air-to-soil transition (ATS) model with a newly proposed dielectric roughness parameterization scheme (Chapter 3). The Tor Vergata discrete scattering model (TVG) integrated with the advanced integral equation model (AIEM) was adopted as the baseline model configuration to simulate the L-band brightness temperature (T_B^p, p =H, V). Then, the ATS model was coupled with the foregoing model to assess its performance. The comparison results indicate that the consideration of the ATS model was necessary for L-band radiation modeling. In Chapter 4, using the coupled enhanced physically-based discrete scattering-emission model, namely, the ATS+AIEM+TVG model (Chapter 3) with the community land model (CLM) v4.5 in a DA system, the soil physical properties were retrieved via assimilating SMAP T_B^p. The results revealed an improvement of the estimates of the soil property profile, which highlights the potential of using the DA system developed in this thesis for soil property to obtain regional and even global soil parameter sets consistent not only in physics but also at different scales. Moreover, we should further investigate the uncertainties in LSM and L-band radiometry modeling, because this may continue to improve not only the retrieval of soil properties but also the estimation of land surface states/fluxes.