Filtering is a key step in the processing of airborne LiDAR point cloud data, and the morphological filtering algorithm has long been considered a classic and effective filtering algorithm for airborne LiDAR point cloud. Because the traditional morphological filtering algorithm retains poor terrain characteristics, which affords poor filtering results, a new morphological filtering algorithm based on thin-plate spline multilevel interpolation was proposed. In this algorithm, the filtering window was considered to reduce continuously during the morphological open-operation process and the thin-plate spline interpolation under different windows was used for processing. This process was iterated from top to bottom until the window size was smaller than the set minimum filter window size. Experiments were conducted using the test data set provided by the International Society of Photogrammetry and Remote Sensing. Results show that the accuracy of the proposed algorithm increases significantly, the filtering effect improves in areas such as buildings or slopes, and topographic features are effectively retained.