This paper aims at providing a framework for detection and diagnosis of the performance of a combinational feedforward (FF) and cascade (CC) control system. It is the extension of our previous work [1,2]. The main idea is to extract the only CC effect and the combination of FF with CC effects, respectively. In the only CC effect, the output variances of the primary and the secondary loops can be turned into the cascade-invariant and cascade-dependent terms, respectively. The combination of FF with CC effect can also be decomposed into the cascade/feedforward invariant term, the cascade-invariant/feedforward-dependent term and the cascade/feedforward dependent term. The diagnosis tree based on these decomposition terms is proposed to assess the performance of the FF/CC control system. The sequence of the statistical inference system is developed to diagnose fault causes. The capability of the proposed method is demonstrated via a cascade control system with the feedforward loops and multiple faults.