It is common to see friction induced fire accidents during the storage and transportation of wet granulation sulfur. To study the sulfur ignition mechanism, a new test apparatus was designed and developed to reproduce the friction phenomena at laboratory scale. Tests have been performed under different loads, water contents and materials. After SEM-EDS and XRD examination, the analysis of friction products uncovered the sulfur ignition mechanism. When the normal load was large, heat caused by friction increased the environment temperature. When the temperature reached the spontaneous combustion point of sulfur, fire occurred. Under a low normal load, combustible iron sulfide was produced during long time friction, and the sulfur was ignited by the spontaneous combustion of iron sulfide. When the water content is 20%, sulfur friction ignition is most likely to occur. In the selected pairs of friction materials, the mild steel pair Q235~Q235 is the most difficult to ignite. In light of the test results and the actual situation on site, safety measures preventing friction ignition accidents during the storage and transportation of wet granulation sulfur are proposed and applied in the field.