In order to study the generation mechanism of chaos in flow system, the dynamical mechanism and energy conversion of plane five-mode Lorenz-like equations are studied by moment analysis method. The five-mode system was transformed into a Kolmogorov type system, which is decomposed into three types of torques: inertial torque, dissipation and external torque (drive torque). Through the coupling of different torques, the mechanism of chaos and the relation between energy and external torques were analyzed. Dynamic behavior evolution and energy conversion of the system are analyzed by Casimir function, and the bound of chaotic attractor is estimated. The physical mechanism of flow instability and turbulence is preliminarily explained.