It’s necessary for power transmission system to conservation energy. In the past, overhead transmission lines are often selected based on the economical current density, and checked by the corona, electromagnetic environment and the audible noise, et. al. Owing to the large changing in the technology and the energy price in the national economy system, the technical specifications are outdated. According to the factual condition of national economic development, select the reasonable conductor cross section, which can conservation the energy and reduce the losses. This paper has researched on the method of economical current density, and advance a method through simply calculated to select the conductor cross section, which is checked by the example of 110kV second power supply project in the Jidong Oilfield. The results show that the conductor selected through the economical current density method is not reasonable, and the reasonable conductor cross section is selected by this paper method.