AC filter，as an important part of ultra-high voltage direct current（UHVDC）transmission systems，has such functions as regulating voltage，filtering harmonics and providing reactive power compensation，and its safe and stable operation is very important for DC transmission system. In this paper，the heating of the joint of AC filter capacitor at a ±800 kV converter station is analyzed and summarized. The early detection of the fault is conducted through temperature rise tests and routine test of capacitor. It is identified by dissecting and testing the wiring terminals of the outgoing bushing of the capacitor that weak crimping between embedded bushing connectors and copper strands is the main cause of the heating of the joint. The specific process improvements measures of the bushing joint and corresponding maintenance strategies of capacitor are proposed so to ensure safe and stable operation of filter of UHV converter station.