Firstly, the observational techniques for soil moisture and the current situation of soil moisture databank are summarized, and the different aspects related to soil moisture simulation are carefully investigated, including the discretization schemes for Richards equation, the physical and biochemical processes in land surface model, the intercomparison of land surface models and the estimation of optimal parameters and uncertainties. Secondly, different data assimilation algorithms are reviewed, especially two widely used methods, i.e., 4-Dimensional Variational assimilation(4-D var）and Ensemble Kalman Filter(EnKF), are compared in detail, and also recent progresses in application of land data assimilation are evaluated. Finally, some considerations of future directions of research are discussed. 