The design and development of the TRMS66.4 slag roller mill and the technological characteristics of the slag grinding system are introduced. TRMS66.4 mill adopts high-efficiency cage-shaped structure separator, which has high sorting efficiency; adopts modular design to realize multiple combinations and improve design efficiency; and uses ductile iron, which can improve quality of product; adopts finite element analysis to ensure the reliability of key components of the equipment; at the same time, it is equipped with a remote online monitoring and diagnosis system. The operation of TRMS66.4 slag roller mill in Fangcheng Gang, Guangxi for grinding slag and composite powder shows that，when the specific surface area of slag powder is 420m²/kg, the output is 290t/h, and the power consumption is 33.5kW·h/t; when the specific surface area of composite power is 450m²/kg, the output is 280t/h, and the power consumption is 36.5kW·h/t.