In this research, we report the effect of humidity and film thickness on the photoemission, optical and morphological properties of CsI thin films prepared by the thermal evaporation technique. The absolute quantum efficiency of CsI thin films has been obtained by using photoemission measurement in the spectral range of 150 nm-200 nm. The degradation in quantum efficiency of CsI thin films under the influence of humidity has been presented. The optical reflectance and transmittance of CsI thin films have been measured directly in the 120 nm-240 nm spectral range. The optical constants have been found to vary with film thickness. The transparency of CsI films has been decreased after exposing to humidity. In contrast, the reflectivity has been increased. The optical band gap energy has been calculated by using Kubelka-Munk (K. M.) model and from first derivative of absorbance. It has been found that the band gap energies obtained from the first derivative of absorbance are slightly higher than those obtained from K. M. model. The average grain size obtained from scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques are observed to be almost similar (similar to 170 nm) for the thinner films, however for the thicker films the average grain size obtained by SEM technique is three times larger (similar to 1500 nm) than those obtained by AFM technique (similar to 550 nm). Various morphological parameters such as maximum peak to valley height, average height, average roughness and root mean square of CsI thin films analyzed from AFM technique are observed to increase after exposing to humidity.