The problem of modeling and fault diagnosis of the single loop and cascade loop control systems was studied based on the deep quantitative knowledge model

signed directed graph (SDG）.A bidirectional inference method based on supposition and validation was introduced

and the problem of the shielding of the multiple control loops in the complex system with respect to the system fault in the traditional fault diagnosis was resolved.The method of fault diagnosis based on the SDG model was applied to the actual distillation equipment under atmosphere pressure

which proved the validity and feasibility of the SDG fault diagnosis method in the complex systems with multiple control loops.