The crystal structure, electronic properties, and optical properties of 4H-SiC were investigated using first-principles calculations based on density functional theory (DFT) under high pressure. By analyzing the variations in relative volume, Si—C bond length, and structural energy of 4H-SiC across different pressures, it is found that the structure remains stable without any phase transitions up to 70 GPa. Beyond 70 GPa, the RS structure with metallic characteristics becomes energetically more favorable. Interestingly, as pressure increases, the semiconductor bandgap of 4H-SiC shows an unexpected widening trend. Concurrently, significant changes occur in its optical properties, including absorption characteristics, dielectric function, and refractive index, highlighting the potential of pressure to finely tune the electronic and optical properties of 4H-SiC. This study not only confirms the remarkable physical properties and application potential of 4H-SiC under extreme high pressure，but also provides a theoretical foundation for its use in high-pressure optoelectronic devices.