Adult stem cells or residential progenitor cells are critical to maintain the structure and function of adult tissues (homeostasis) throughout the lifetime of an individual. Mis-regulation of stem cell proliferation and differentiation often leads to diseases including cancer, however, how wildtype adult stem cells and cancer cells respond to cellular damages remains unclear. We find that in the adult Drosophila midgut, intestinal stem cells (ISCs), unlike tumor intestinal cells, are resistant to various cellular damages. Tumor intestinal cells, unlike wildtype ISCs, are easily eliminated by apoptosis. Further, their proliferation is inhibited upon autophagy induction, and autophagy-mediated tumor inhibition is independent of caspase-dependent apoptosis. Interestingly, inhibition of tumorigenesis by autophagy is likely through the sequestration and degradation of mitochondria, as compromising mitochondria activity in these tumor models mimics the induction of autophagy and increasing the production of mitochondria alleviates the tumor-suppression capacity of autophagy. Together, these data demonstrate that wildtype adult stem cells and tumor cells show dramatic differences in sensitivity to cellular damages, thus providing potential therapeutic implications targeting tumorigenesis.