Gaussian process (GP) models encounter computational difficulties with large spatial datasets since its computational complexity grows cubically with sample size n. Although the Full-Scale Approximation (FSA) using a block modulating function provides an effective way for approximating GP models, it has several shortcomings such as the less smooth prediction surface on block boundaries and sensitiveness to the knot set under small-scale data dependence. To address these issues, we propose a Smoothed Full-Scale Approximation (SFSA) method for the analysis of large spatial dataset. The SFSA leads to a class of scalable GP models, whose covariance functions consist of two parts: A reduced-rank covariance function capturing large-scale spatial dependence and a covariance adjusting local covariance approximation errors of the reduced-rank part both within blocks and between neighboring blocks. This method can alleviate the prediction errors on block boundaries; it also leads to more robust inference and prediction results under different dependence scales due to better approximation of the residual covariance. The proposed method provides a unified view of approximation methods for GP models, encompassing several existing computational methods for large spatial datasets into one common framework, including the predictive process, the FSA, and the nearest neighboring block Gaussian process methods, allowing efficient algorithms for more robust and accurate model inference and prediction for large spatial datasets in a unified framework. We illustrate the effectiveness of the SFSA approach through simulation studies and a total column ozone dataset.