Six new acyclic diterpenoids named Aphanamixins A-F (1-6), together with two known compounds of nemoralisin and nemoralisin C, were isolated from the stem bark of Aphanamixis polystachya (WALL) J. N. BARKER. Their structures were established through a comprehensive analysis of NMR spectroscopic data and high resolution mass spectrometric data. The absolute configurations of carbon stereocenters were determined by means of auxiliary chiral α-methoxy-α-(trifluoromethyl)phenylacetic acid (MTPA) derivatives and circular dichroism (CD), respectively. All the new isolates were tested for their antiproliferative activity against HepG2, AGS, MCF-7, and A-549 cancer cell lines and they exhibited weak cytotoxicities (IC50>10 µM). Moreover, we highlighted that the six new diterpenoids characterized by acyclic skeleton was rarely seen in nature.