Domain wall dynamics under the joint action of linearly polarized microwave magnetic field and spin transfer torque was analysed in terms of the domain wall collective coordinates.It was confirmed that steady domain wall motion can be sustained with the assistance of microwave magnetic field,even in the presence of the adiabatic spin transfer torque only.Approximate analytical solution to the equation of motion showed that the steady domain wall motion is derived from the nonlinear oscillation in domain wall width excited by the microwave field.