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Objective To investigate the molecular mechanisms underlying the effect of homoharringtonine (HHT) on the proliferation and apoptosis in HT29 human colorectal tumor cells. Methods HT29 cells were treated by 0, 0.007812, 0.015625, 0.03125, 0.0625, 0.125, 0.25, 0.5, 1, 2, 4, 8 μg/mL HHT for 24, 48 and 72 hours. CCK-8 assay was used to assess the cell viability. Colony formation assay was performed to detect the cell proliferation ability. Flow cytometry was used to analyze cell apoptosis. Hoechst33258 fluorescent staining was used to observe the morphology of the cell nuclei. The real-time quantitative PCR was used to detect the mRNA expressions of BAX, Bcl2, caspase-3, caspase-9, mammalian target of rapamycin (mTOR), PI3K, pyruvate dehydrogenase kinase-1 (PDK1), protein kinase B (AKT), raptor, rictor. The protein levels of Bax, Bcl2, pro-caspase-3, cleaved caspase 3 (c-caspase-3), pro-caspase-9, cleaved caspase-9 (c-caspase- 9), poly(ADP-ribose)polymerase (PARP), cleaved PARP (c-PARP), mTOR, raptor, rictor, PI3K, PDK1, AKT, p-AKT were detected by Western blotting. Results Compared with the control group, the proliferation of HT29 cells was inhibited when treated with HHT. Meanwhile, the nuclear fragmentation, chromatin condensation, and apoptotic body of the cells could be observed. Treatment of HHT could increase the mRNA expressions of BAX/Bcl2, caspase-3, caspase-9 and raptor, and decrease PI3K, AKT and rictor in the HT29 cells. The protein levels of pro-caspase-3, pro-caspase-9, PARP, PI3K, PDK1, AKT, mTOR, and rictor were down-regulated, and the c-caspase-3, c-caspase-9, c-PARP, BAX and raptor were up-regulated. Conclusion HHT has the function of inhibiting the HT29 cell proliferation and inducing its apoptosis by blockage of mTOR signaling pathway.