Incoherent scattering spectrum of a collisional plasma with an arbitrary velocity distribution function is presented in this paper. Two integrals with complex singular points have been solved in order to obtain the results. The incoherent scatter spectrum during HF heating in low ionosphere region is computed. The effect of collision frequency, non-Maxwellian index, electron density, electron temperature and ion temperature on power spectrum is discussed. Under the action of collision frequency and non-Maxwellian factor, the original relationship between ionospheric parameters and the characteristics of spectral shape is invalid, which will bring about a large error in the inversion of ionosphere parameters, thus the original theoretical model must be revised.