A low-temperature firing process is available for cost saving to produce a multilayer chip ZnO varistor containing pure silver (Ag) formed as internal electrodes and calcined at ultralow firing temperature of 850-900° C., which process comprises: a) individually preparing ZnO grains in advance doped with doping ions for promotion of semi-conductivity of ZnO grains if calcined; b) individually preparing a desired high-impedance sintering material to be fired as grain boundaries to encapsulate ZnO grains; c) mixing the doped ZnO grains of Step a) with the high-impedance sintering material of Step b) in a predetermined ratio to form a mixture and proceeding with an initial sintering to have the mixture sintered and ground as composite ZnO ceramic powders, and d) processing the sintered mixture of Step c) to make multilayer chip ZnO varistors containing pure silver (Ag) internal electrodes but sintered at ultralow firing temperature of 850-900° C.