Self-consistent equation for order parameter of nematic liquid crystal (LC) in solution is obtained on the basis of Lebwohl-Lasher model and mean-field approximation. Numerical calculation shows that LC in solution with different volume fractions all undergo the first-order phase transition at 2> =0.324 without external fields, and TN1 is proportional to the volume fraction, TφN1= Tφ=1N1φ. Under applied external fields, LCs undergo second-order phase transition at 2 > = 0.40. Distribution curves are in Ⅴ shape. The variation of the distribution mean square errors with> has a maximum at 2> = 0.20.