Halide perovskites are promising photovoltaic, solar cell, and semiconductor materials. Density-functional theory (DFT) models address compressive and tensile biaxial strain effects on APbCl 3, where A=(K, Rb, and Cs). This research shows how A-cation impacts bandgap energy and band structure. The direct bandgap for KPbCl 3, RbPbCl 3, and CsPbCl 3 is found 1.612, 1.756, and 2.046 eV, respectively; increases from A= K to Cs. When spin–orbital coupling (SOC) is introduced, bandgaps in KPbCl 3, RbPbCl 3, and CsPbCl 3 perovskites are reduced to 0.356, 0.512, and 0.773 eV, respectively. More tensile strain widens the bandgap; compressive strain narrows it. Without SOC, the bandgaps of KPbCl 3, RbPbCl 3, and CsPbCl 3 were tuned from 0.486 to 2.213 eV, 0.778 to 2.289 eV, and 1.168 to 2.432 eV, respectively. When the compressive strain is increased, the dielectric constant of APbCl 3 decreases (redshift) and increases (blueshift) as the tensile strain is increased. Strain improves APbCl 3 perovskite's optical performance.