In this paper,new boundary differential equations for the two-dimensional exterior scattering problem have been derived.It has been shown that the Helmholtz equation can be reduced to an inhomogeneous Bessel's equation in a body-fitted coordinate system.By imposing the Sommerfeld radiation condition to the general solution of the Bessel's equation,an integro-differential equation,which is equivalent to the original Helmholtz equation,can be obtained.The boundary differential equation can then be established by use of the integration by parts to get rid of the integral in the integro-differential equation for high frequency problems.Numerical examples have been presented to demonstrate the validity of the new boundary differential equations.