Based on the Lorenz model derived from the equations of hydrodynamics of nebula, we discuss the formation of binary stars by the qualitative analysis theory of nonlinear equation. Here the two wings in the Lorenz model form just the binary stars, whose Roche surface is result of evolution under certain condition. The nonlinear interaction plays a crucial role, and is necessary condition of the formation of binary stars and of multiple stars. While the linear equations form only a single star. Moreover, the Lorenz model may be obtained from synergetics yet, and some dynamical equations of the order parameter can also form the binary star system.