Lemons Model made by Akerlof.G is very important in information economics.In reality,various buyers' remarks on cars of the same quality are different.Some random factors,which influence buyers' utilities for cars of the same quality,are considered for the problem solving.Assuming the general quality of a car,the paper makes necessary complement to the old car market model,then conducts a mathematical analysis by improving the model,draws a conclusion that the balanced trade amount is decided by evaluation parameter of buying and seller,which is consistent with reality.