Abstract: The China ADS Front-end demonstration facility(CAFe) is the prototype accelerator for the China Accelerator Driven Subcritical system(ADS), and exhibits extremely high requirement on operation stability. During the operation of CAFe, the field emission effect interferes with the pickup signal from the superconducting cavity, which is one of the most common causes for beam trips and other operation faults. Such interference seriously harmed the accelerator operation stability. The origin of the problem comes from the fact that field emission electrons cause discharge on the microwave transmission device, which interferes with the signal transmission and causes the low-level radio frequency system to control the superconducting cavity incorrectly, and finally leads to the superconducting cavity failure and accelerator shutdown. In this paper, the structure of the pick-up antenna was optimized to avoid the field emission electrons from entering the pick-up coupler, so as to solve the problem of abnormal signal. The optimized antenna showed desirable results during the online operation tests, the problem of abnormal signal extraction was solved, and the stability of accelerator operation was improved.