The CdS thin film is the most important narrow band gap n-type layer for efficient optoelectronics applications. Here, we report CdS thin film on silicon and glass substrates through direct current(dc) magnetron sputtering technique. The effect of air-annealing on deposited thin films & its impact on morphological and optoelectrical characteristics of CdS thin films have been explored in this study. The sputtered CdS thin films were characterized using the scanning electron microscopy(SEM), atomic force microscopy(AFM), diffuse reflectance spectrophotometer and solar simulator system. FESEM images show the uniform, homogeneous and crack free grain, AFM topography exhibits the roughness increases with increasing air-annealing. EDS spectra confirm the elemental composition of Cd and S in deposited thin film. The optical analysis showed the reduction of energy gap from 2.36 eV to 2.29 eV and the estimated Urbach tail increases with increasing air-annealing. Moreover, the complex reflective index, dielectric constant, tangent loss, volume energy & surface energy loss studies were effectively influenced by the air-annealing conditions. The impact of air-annealing on the performance of Ag/Ni: CdS/Si/Al photodetectors are analyzed through I-V measurement at room temperature. The detector parameters such as responsivity (R) detectivity (D*) snd quantum efficiency have been estimated and discussed in detail. Here the responsivity and detectivity decreases and quantum efficiency increases with increasing air-annealing. The responsivity, and detectivity of CdS film are in the range of 0.42 and 0.46 AW-1 and 2.58 x 1011 to 4.09 x 1011Jones, respectively. The effect of the air-annealing was noticeable and obviously improved the detection parameters of the designed photodetectors device.