中国物理学会高能物理分会第十三届全国粒子物理学术会议（2021）

Due to the large size of 20 inch MCP PMT, the geomagnetic field have big influence on the performance of PMT, including time response and detect efficiency. A Permalloy shielding had been installed on all 20 inch MCP-PMT. Permalloy can change the direction of the magnetic field because of its higher permeability compared to ordinary steel. Since the magnetic field is guided through a low magnetic resistance path, it ensures that the PMT is not affected by the geomagnetic field. In this work, we will report on the application of magnetic shield on 20 inch MCP-PMT at LHAASO-WCDA.