The stability of steel slag is the key technical bottleneck restricting its utilization as building material. We compared the stability of rotating furnace slag (RFS),Hot Splashing Slag(HSS),and Stewed Detained Slag(SDS) obtained by different testing methods—autoclave expansion rate test, autoclave pulverization rate test,grading autoclave expansion rate test,as well as hot water curing under different temperatures. Our findings revealed that the stability results of MR-HSS and ER-SDS obtained by autoclave expansion rate method and autoclave pulverization rate method were contradictory. The autoclave pulverization rate was closely related to the change of f-CaO, MgO content, and MgO/Fe₂O₃ mass ratio before and after autoclave test. The uneven distribution of f-CaO and f-MgO presented high potential risk to the stability based on the relationship between stability and chemical components changes before and after autoclave testing. The volume instability of MR-RFS would be aggravated when steel slag was grouped into discontinuous grades; as a result, steel slag with consistent gradation should be preferred for use as fine aggregates. Therefore, in consideration of the sensitivity of testing results and the safety of utilization, we suggest to adopt the autoclave expansion rate testing method in testing the stability of steel slag used as fine aggregates. In addition, the autoclave expansion rate in association with appearance integrity can be taken as comprehensive evaluation indicators.