The postal transportation network is critical in the postal enterprise operation, and the borough transportation network is its fundamental part. This paper systemically analyzes the postal transportation problem in terms of its optimization objectives, ranges, vehicle factors, time limit and frequency, cost, and so on. And then the paper proposes the corresponding lexicographic order linear programming model based on the characteristics of borough postal transportation network to minimize fixed costs and transportation costs. The optimal solution is derived using genetic algorithm. Simulation results indicate that the model has significant guidance to adjust and improve current enterprise's operational mode.