Low-dimensional organic-inorganic hybrid perovskite materials have attracted extensive attention due to their exotic photoelectric performance. In this paper, different size (CH3NH3PbI3) MAPbI3 nanocubes were studied by using diamond anvil cell (DAC), under high-pressure conditions. And explored the regulation of the size effect on the the optical properties of MAPbI3 nanocubes under high pressure. The in-situ high-pressure UV-visible absorption and photoluminescence spectra showed that the band gap and optical properties of MAPbI3 nanocubes with two different sizes showed different changes. The band gap of small-sized MAPbI3 nanocubes decreases with the increase of pressure below 0.25 GPa, while for large-sized nanocubes, the band gap continues to increased with the increasing of pressure below 0.67 GPa. The in-situ high pressure Raman measurement and analysis showed that the size effect was related to the interaction between the octahedron ［PbI6］4- and the organic cation CH3NH3+. The results provide a research basis for in-depth understanding the band gap regulation, optical properties and structural stability of low dimensional organic-inorganic hybrid perovskite nanocrystals, and it provides a research idea for improving the conversion efficiency of organic-inorganic hybrid perovskite cells.