The relationship between ambient relative humidity H and the position shift of a spectral line was in-vestigated both experimentally and theoretically. An echelle-based ICP emission spectrometer equipped with a CID detector was used for experimental verification of the derived model. The shift of a spectral line is quanti-tatively described by two defined spectral shift functions. △λx (x,λ, H) (in the x direction of the CID detec-tor) and △λy(y, λ, H) (in the y direction of the CID detector). Experimental results indicate that △λx(x, λ, H) does not change with a variation in ambient relative humidity, but △λy(y, λ, H) does. A spectral shift e-quation, i.e. an empirical second-order polynomial equation, can be used to describe the relationship between △λy(y, λ, H) and H. Based on the classical dipole model, classical mechanics and electrodynamics the empiri-cal spectral-shift equation involving △λx(y, λ, H) and H was theoretically deduced. The theoretical result is in good agreement with the experimental findings. The theoretical results indicate that the coefficients of the em-pirical spectral-shift equation are related to the basic physical parameters of materials and the geometric config-uration of the echelle CID ICP-AES, and also provide physical meaning to the coefficients of the empirical shift equation obtained experimentally.