The seeds of Caesalpinia minax Hance have shown anti-tumor potential, while the chemical principle is still unknown. In a search for anti-tumor compounds, six new cassane-type diterpenoids, 12-demethylcaesalpin G (1), caesalpinolide H (2), 12-demethylcaesalpin H (3), caesalpinolide J (4), 12-O-ethyl neocaesalpin B (5), and 3-deacetyldecapetpene B (6), were isolated from the seeds of C. minax Hance, along with fifteen known analogues. The structures of the new compounds were established by means of spectroscopic techniques (NMR, HRESIMS and IR). The absolute configurations of the new compounds were determined by their ECD spectra. All of the new compounds were tested for their anti-proliferative activity against human lung cancer A549 cells, breast cancer MCF-7 cells, and ovarian cancer HEY cells. The results indicated that only compound 6 displayed moderate cytotoxicity against three cancer cell lines. Thus, the opening of furan ring in cassane-type diterpenoids might enhance the cytotoxic activity.