The corrosive behavior of the nickel-based alloy G3 at room temperature (25℃) and high temperature (90℃) in a complex medium environment containing H2S, CO2, Cl- with various states of elemental sulfur was studied by using electrochemical microscope (SECM), scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS) in this paper, and the influence of elemental sulfur was analyzed. The results show that the corrosion becomes more serious with increasing temperature and slight pitting occurs on the surface of the nickel-based alloy G3. The anode polarization curve has a passivation region at room temperature, but the active dissolution appears and passivation disappears at a temperature of 90℃. The phases were analyzed that the passivation film formed on the surface of the nickel-based alloy G3 is mainly composed of oxides of Ni, Cr and Fe. The passivation film is dissolved due to the intrusion of S2-, and the corrosion products were composed of NiS and FeS2. The corrosion resistance of G3 alloy is weakened in the sulfur-contained environment, and the suspended sulfur has the most dense passivation film and the least corrosion rate compared with the deposition of sulfur and the precipitation of sulfur. Studies have shown that sulfur is a strong catalyst, and its existence leads to severe local corrosion. The state of elemental sulfur is a key factor affecting the compactness and growth rate of corrosion product membranes and the corrosion rate of alloys.