Abstract: A non-linear and non-hydrostatic numerical model is designd to study the development and evolution of symmetric instability. The test results show that structure and evolution of non-linear symmetric instability are very different from linear. When the basic state is symmetric instabile, in the linear symmetric instability, slantly symmetric circulations are maintained thoughout the growth of perturbations. However, in non-linear symmetric instability, it is very similar with the former in early time of perturbations growing. With growth of the perturbations, non-linear interaction of advection term happens and the symmetric circulations are destroyed.