Ladar point-cloud data filtering is basis and critical steps of the ladar data processing. Filtering algorithm based on mathematical morphology is applied widely. But the existing filtering algorithms have some problems when processing point-cloud data which contains some blank region of large area. To solve these problems, an improved progressive multi-scale mathematical morphology filtering algorithm is proposed. It deals with the blank region by improving morphological opening operation. According to basic principles of mathematical morphology, feasibility of improved morphological opening operation is proved. Then basic steps of the proposed algorithm are elaborated, and performance characteristics are analyzed. The proposed algorithm is verified experimentally by using simulation data and public test data. Experimental results show that the proposed algorithm can perfectly filter point-cloud data which contains some blank region of large area, and it has better filtering performance compared with other representative filtering algorithm in most cases.