The construction of sensitive and durable sensitizers facilitates the development of metal oxide semiconductor (MOS)-based sensors with excellent gas sensing performance. Here, amorphous ruthenium oxide (RuO_x) doped with lithium ions was successfully synthesized, and employed as a sensitizer to enhance the gas response of ZnO sensing materials. The gas sensing test results show that amorphous RuO_x has better sensitization effects on ZnO than crystalline RuO₂ at an optimum working temperature of 210 °C, such as a higher response, lower working temperature, better selectivity and faster response/recovery speed. The response of the RuO_x/ZnO-1 sensor remains almost unchanged over a period of 60 days, while the RuO_x sensitizer loaded on ZnO retains an amorphous structure after 60 days of working. Ex situ XPS also demonstrates that amorphous RuO_x does not crystallize into polycrystalline RuO₂ at 210 °C. The enhanced gas-sensing performance of the RuO_x/ZnO-1 sensor can be attributed to the abundant active sites and better catalytic performance of amorphous RuO_x, as well as enhanced electronic transmission between RuO_x and ZnO. This is the first report showing that amorphous RuO_x can work stably as a sensitizer at high temperature (210 °C), which facilitates the widespread use of amorphous metal catalysts in various fields.