In view of the appropriate physicochemical characteristics and environmental friendliness of the trans-1,1,1,4,4,4-hexafluoro-2-butene (HFO-1336mzz(E)) substance, the thermal-decomposition mechanism as well as the fire-extinguishing mechanism and performance of this agent were systematically studied by employing both experimental and theoretical methods in this work. We found that the HFO-1336mzz(E) agent not only has promising thermal stability at room temperature but also exhibits pronounced fire-extinguishing performance, which is comparable to that of HFC-236fa and even better than that of HFC-125 extinguishant. Additionally, the promising fire-extinguishing performance of HFO-1336mzz(E) may result from the physical and chemical extinguishing effect of its thermal-decomposition products including HFO-1336mzz(Z), HC equivalent to CCF3, CF3C equivalent to CCF3, and CF3H, which makes a significant contribution to capturing the free radicals in the flame, as well as cooling and diluting the combustible fuel-air mixture. Both experimental and theoretical results suggest that the HFO-1336mzz(E) agent is a highly recommendable candidate for Halon extinguishant, which is worthy of further investigation and evaluation of its practical applicability in fire-suppression utilization.