Interface stress and contact resistance play key roles in the safety and stability assessment of non-insulated superconducting pancake coils. An electrometric method for the interfacial stresses and contact resistance of multi-turn coils of different materials has been established, which is further applied to the measurement and analysis of contact stresses and resistances of the composite superconducting coils under the extremely low temperature environment. The mechanical and electrical behaviors are coupled through an extended electro-mechanical contact model, which also reveals the electro-mechanical interaction mechanism of the coil. The extended contact model was verified by comparison with experimental results, and the proposed electrometric method was verified by comparing the interface stresses calculated by two approaches. The contact stresses and resistances of superconducting coils with different turns are successfully obtained through the proposed electrometric method, which provides bases for the evaluation of the transport and mechanical performance of superconducting coils.