The existing insulator detection methods are difficult to meet the requirements of safe operation and upgrade optimization of the power grid. ZnS:Cu electroluminescent composite materials possess the characteristic of electric field visualization, offering prominent advantages such as intuitiveness, convenience, and low cost in power grid insulator detection. However, their luminescence mechanism remains unclear. This study synthesizes “ZnS:Cu composite materials” by blending and curing ZnS:Cu with a matrix binder. Four sets of experiments were designed to elucidate the microscopic mechanism: (1) Ultraviolet irradiation experiment validates its energy injection characteristic; (2) Insulating oil pressurization experiment demonstrates that the composite exhibits both intrinsic and injection luminescence properties; (3) Jet-driven excitation experiment proves that high-energy electrons are the direct cause of luminescence; (4) Rod-plane electrode corona experiment reveals that luminescence under atmospheric conditions is dominated by the injection mechanism. These findings provide theoretical guidance for the subsequent design of ZnS:Cu composites and the regulation of their luminescence properties.