A large amount of ferronickel slag containing high MgO composition is discharged every year in China, but it has not found an efficient method to utilize the slag so far. We investigated the feasibility and the effect of using ferronickel slag containing high MgO composition as an active mixed material on the basis of analyzing its composition and performance. The results show that the content of amorphous mineral in the slag is 88.1%. The content of MgO reaches 27.07% and it mainly exists as enstatite and hortonolite. The slag's activity and its dosage used in the cement are affected by its specific surface area. As an active mixing material, its specific surface area should be no less than 454.6 m²/kg. The bigger the specific surface area of the slag is, the higher the active index is, and the larger the dosage in the cement is. The autoclave soundness of the cement mixed with high-MgO ferronickel slag is qualified. Even if the mixing dosage of the slag with specific surface area of 842.9 m²/kg is up to 50%, the autoclave expansion rate of the new cement is only 0.11%, which is significantly lower than 0.5% as required by GB750-92. Therefore, ferronickel slag containing high MgO composition can be used as an active mixed material.