Based on the Heisenberg model, the anti-ferromagnet following the interaction suddenly switched-on at t=0 is considered in this paper. In the limit of low temperature, the time evolution of observables is analytically studied by utilizing the flow equation method. According to the unique feature of the system, the way to obtain the evolution of observables is shown and applied to the system we focused on. Further, special emphasis is laid on the application of the flow equation method to non-equilibrium magnetic systems so that the validity of the method applied to this kind of systems is traced. By means of this method, the evolution of magnetization of the ground state is traced in detail. It is found that the magnetization oscillates with time rather than converges,which is different from that of non-equilibrium Luttinger liquids.