Investigation of the entomogenous fungus Fusarium equiseti LGWB-9 from Harmonia axyridis led to the isolation of fusarisetin B (2) and its analog, fusaketide A (1), along with two known compounds (3 and 4). Among them, fusaketide A (1) represent the first example of natural polyketide carbon skeleton with a [6/6/5/5] tetracyclic ring system. The planar structure and relative configuration of 1 was established on the basis of NMR spectroscopic data and 13C NMR chemical shift calculation. The absolute configuration of 1 was assigned by quantum chemical TDDFT calculation of its ECD spectrum and single-crystal X-ray diffraction analysis using Cu Kα radiation. Compounds 1 and 2 showed cytotoxicities against MCF-7, MGC-803, HeLa and Huh-7 cell lines with the IC50 values ranging from 2.4 to 69.7 μg ml-1. Cell invasion, migration, DAPI staining, and flow cytometry experiments were carried out to examine the effect of 2 against MGC-803 cells. Western blot results showed that 2 could induce MGC-803 apoptosis through up-regulation of Bax and down-regulation of Bcl-2.