Two-dimensional IV-VI chalcogenide compounds MX (M = Sn, Ge; X = S, Se, Te), as novel semiconductor materials, have attracted extensive attention owing to their suitable band gaps and excellent optoelectronic properties.In this study, we systematically investigated the effect of layer number on the electronic band structure, density of states (DOS), and optical absorption properties of MX materials using density functional theory (DFT).Our results show that the band gap decreases monotonically with increasing layer number, accompanied by multiple direct-indirect band gap transitions Specifically, GeSe transitions from a direct band gap in the monolayer to an indirect band gap in multilayers; GeS transitions from an indirect band gap in the monolayer to a direct band gap in multilayers; and SnTe transitions from an indirect band gap in monolayers through quadrilayers to a direct band gap in the pentalayer. indicating a critical thickness-driven electronic reconstruction. These transitions indicate a critical thickness-driven electronic reconstruction, originating from the rearrangement of band-edge states induced by interlayer orbital hybridization involving M-p and X-p orbitals. DOS analysis further reveals that the near-band-edge states are predominantly derived from hybridized M-p and X-p orbitals.With increasing thickness, the optical response is significantly modified. The absorption edge exhibits a systematic red shift, consistent with band gap narrowing. Meanwhile, the optical absorption coefficient increases from 104 cm^-1 in monolayers to 105 cm^-1 in pentalayers within the visible to near-infrared range. This enhancement is attributed to an increased joint density of states and strengthened optical transition matrix elements driven by interlayer coupling. These findings demonstrate that the optoelectronic properties of MX materials can be tuned by varying the layer numbers, positioning MX as a promising candidate for channel materials in optoelectronic devices in the post-Moore era.