Batteries with increasing energy density have boosted the development of electric vehicles with longer driving ranges but are also arousing safety concerns, especially thermal runaway, which impede their large-scale application. Herein, guided by electro-chemo-thermal process modeling, we reveal the coupled roles of thermal tolerance, thermal conductance, and overheating-response properties of separators in preventing thermal runaway under abusing conditions. As such, we realize the thermal process intensification by integrating these properties into a thermal-managing trilayer separator, by sandwiching a thermal-tolerant poly(p-phenylene benzobisoxazole) matrix between mixture layers of thermal conducting boron nitride nanosheets and overheating-responding polyvinylidene fluoride. Compared with commercial separators, such a separator shows much improved safety performance including nonflammability, anti-shrinkage performance at high temperature (almost zero at 350 °C) and thermal shutdown property, finally doubling safety-responding window for battery management system. The strategy of coupling thermal tolerance, conductance, and overheating-response capability in a separator provides a new venue to intensify the thermal management of batteries and paves up the way for the application of high-safety and high-energy-density batteries.