Amodel of lower ionosphere with non-spherical surface moving horizontally is proposed for wave reflection during sunrise and sunset. Combined with spheric surface for reflaection during day and night, this model can be made quite perfect to explain the well-known diurnal variation of reflection from lower ionosphere. Aconcept about reflection region is presented, upon which the formulae for calculating field-strength of sky wave are modified to be the integral form and they can be applied in more general cases. Finally, the amplitude and phase, as well as their diurnal variation, for sky wave propagation at LFband by one-skip mode on two paths are calculated. One path is from station Mto Qindao, the other is from station, Mto Xinxiane. The results are analysed and compared with the experiments.