<p>In order to explore the leaching law of different elements in the composite cementitious system composed of ferrous extraction tailing of nickel slag (FETNS) and ordinary Portland cement (OPC), element leaching test under different influencing factors was designed with the aid of ICP-OES, XRD, and SEM-EDS. The experimental results show that, with the extension of leaching time, the continuous hydration reaction in the system enables the leaching amount of Si, Al, Mg, and Ca elements to show an overall downward trend. In the alkaline environment, the more sufficient hydration reaction consumes more soluble elements, resulting in a significantly smaller leaching amount than that in the neutral environment. Temperature is also an important factor affecting the leaching of elements. The rise of temperature promotes the dissolution of amorphous phases Si, Al, and Mg in the system, leading to increased leaching amount and higher consumption of C₂S and C₃S, generating more reaction products. In addition, the content and fineness of FETNS also have a significant effect on the element leaching of the composite cementitious system. More importantly, this paper clarifies the leaching safety of internal heavy metal elements when FETNS is used under the above conditions, which provides a scientific guarantee for the safe and efficient application of FETNS in building materials.</p>