This paper presents a design of coupling - in in TE111 double mode cylindrical cavity elliptic function filter in which a coaxial probe is used for excitation. And the radiation field of the driving source is solved by the variational integral approximation method, then the variational expression of the input impedance of the coaxial probe and the expression of the radiation resistance are obtained, the position and the dimension of coupling coaxial probe are solved.It is proved from practice that the coupling design is effective, simple and convenient.