Cooperative transmission has been shown to be able to greatly improve system performance by exploring the broadcasting nature of wireless channels and cooperation among users. This paper proposes a cooperation strategy among rational nodes in a wireless cooperative relaying network as an effort to solve two basic problems, i.e., when to cooperate and how to cooperate. First, a symmetric system model comprising two transmitter nodes (TN) and one destination node (DN) is presented. In this model, each TN plays an equal role and acts as a source as well as a potential relay and has the right to decide the amount of power it should contribute for cooperation. Second, referring to the evolutionary game algorithm (EGA), the above problems are formulated as an optimal problem under certain constraints. Then, a cooperative strategy based on the EGA is proposed, in which TNs will cooperatively work and each will share a certain fraction of its power for relaying. With the moving of nodes, the EGA can adjust the power allocation parameters adaptively. Simulation results demonstrate that when cooperation takes place, the overall system transmission rate will benefit from the proposed strategy and those nodes with good channels will spend more power to cooperate with others.