With the development of wireless mobile network, 3G and 4G technologies are more and more mature nowadays. In the case of public network, it has been a very important part to use RTU (remote terminal unit) for data exchange in our daily lives. Data communication is the main energy consumption part during the whole transmission process. As the main the communications parameter, packet size directly affects the RTU energy consumption. In order to save energy and extend battery life, this paper proposes a packet length adaptive algorithm ASPSA(adapted simultaneous perturbation stochastic approximation) based on SPSA, which can calculate the most effective payload length and obtain the most optimal packet length. The results of theoretical analysis, simulation and spot tests show that the algorithm aroused in this article can improve the energy efficiency of wireless data transmission terminals greatly.