To ensure the fair exchange of buyers and sellers，the fairness of mobile payment protocol was formally analyzed with the theorem of unsolicited test and the existence of sending edge. It was pointed out that the mobile payment protocol has security shortages on fairness. By adding timestamps to the original protocol and changing entity of some messages，an improved protocol was proposed to overcome the security shortages. The improved protocol was proved to satisfy with the fairness under the premise of not affecting performance by the extended strand space model. The research result indicated that the proposed approach can analyze the fairness of protocol efficiently.
