This paper is based on the resonance phenomenon of a square flat no leads package device when it is applied in Ku band, which leads to circuit insertion loss of more than 3dB and serious deterioration of circuit flatness. Based on this phenomenon, circuit resonance analysis is carried out and solutions are explored. In this paper, the RF characteristics and equivalent circuit of coplanar waveguide with three types of defected ground structures are theoretically analyzed. Based on the coplanar waveguide theory, the structural parameters of the third type of defected ground structure and the characteristics of the transmission line with ground cavities at both ends of the transmission line are simulated and analyzed by CST simulation software. The research shows that the RF output pin of QFN package device is easy to form a defective coplanar waveguide with gaps with the surrounding pins and the surrounding ground, which makes the circuit equivalent to an LC parallel resonant circuit. In addition, the length of cavity l, the width of cavity w, the width of gap g, the distance from the gap to the side wall of the cavity d and the simultaneous presence of cavities at both ends of the transmission line will affect the characteristics of the transmission line, making the circuit prone to resonant points in the low frequency band. In this paper, the circuit is optimized and the resonance phenomenon is effectively eliminated by removing the gap g in the equivalent defect ground coplanar waveguide structure in the circuit. Then the method of eliminating resonance phenomenon and circuit design method adopted through theoretical and simulation analysis can effectively improve the flatness in the microwave band. This method can be widely used for reference in the design of modern high-density integrated fourth generation microwave circuits.