The relationship between ambient relative humidity H and the position shift of a spectral line was investigated 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 quantitatively described by two defined spectral shift functions: Δλx(x, λ, H) (in the x direction of the CID detector) and Δλy(y, λ, H) (in the y direction of the CID detector). Experimental results indicate that Δλx(x, λ, H) does not change with a variation in ambient relative humidity, but Δλy(y, λ, H) does. A spectral shift equation, i.e. an empirical second-order polynomial equation, can be used to describe the relationship between Δλy(y, λ, H) and H. Based on the classical dipole model, classical mechanics and electrodynamics the empirical spectral-shift equation involving Δλy(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 empirical spectral-shift equation are related to the basic physical parameters of materials and the geometric configuration of the echelle CID ICP-AES, and also provide physical meaning to the coefficients of the empirical shift equation obtained experimentally.