In present paper, the Chapman's grazing incidence function varing with heights of observational point and solar zenith angle is numerically calculated. The variation of Chapman's function in a realistic model atmosphere due to high temperature gradient, molecular and eddy diffusion transport, gravity field as well as solar activity is computed and discussed for the lower thermosphere. The results obtained indicate, in a realistic model atmosphere of the earth, especially at the thermospheric heights up to 150 km, the Chapman's function differs sufficiently from those calculated by previous authors using the constant scale height and constant scale height gradient models, and the influence of the high temperature gradient in the lower thermosphere plays a leading role among the other effects. The influences of solar activity on Chapman's function appear to be efficient at large solar zenith angles(in a range of 10-40% between high and low solar conditions). These results are of obvious benefit for the studies of solar radiation-photochemistry-dynamics processes in the thermosphere.