The assembling capacitor is an important reactive power compensation equipment in substation and its heating defects seriously affect safe and stable operation of the power grid. In this paper，a disassembly inspection on a typical heating defect in the bushing joint of assembling capacitor is conducted. It is found that there are a few unreasonable designs in structural design of the bushing joint，such as water immersion in the groove of bushing flange，primary involvement of aluminum rain cover，weak contact with bushing flange and guiding screws，serious oxidization of contacting surface due to long term flow，leading heating of bushing joint. As for such heating defect，the effective preventive measures and daily maintenance and suggestions are proposed in this paper so to improve reliability and safety of the reactive system of the power grid.