The current work presents a theoretical investigation of a CdS/ZnSe quantum well infrared photodetector (QWIP). Many body effects are incorporated for the accurate estimation of confined subband energies in the conduction band. Results show that optical transition energy is obtained from short to mid-wavelengths (2–5.85 µm) by tailoring CdS well layer. Rate equation is solved for the photocurrent calculation of the CdS/ZnSe QWIP. Photocurrents of 0.084, 0.11 and 0.14 A/cm2 are observed at 4.3, 5 and 5.85 µm respectively for 20 QW periods. An improved responsivity of 160 mA/W at 5 µm is obtained for high value of doping concentration.