A new analysis method based on node-voltage increments relation function (NVIRF) for soft fault diagnosis of analog circuits is described. In linear analog circuits, the NVIRF is a linear function, thus the linear relativity coefficient is invariant as fault feature. Therefore,a unified fault feature for both hard fault (open or short fault) and soft fault (parametric fault) is extracted. In nonlinear analog circuits with nonlinear two-terminal components, according to the substitution theory and another important character: high-rank fault covers low-rank fault, this paper presents a soft fault diagnosis dictionary approach in order that the NVIRF method can be extended from diagnosing faults of linear analog circuits to those of nonlinear analog circuits. Furthermore. in analog circuits with closed-loop operational amplifier (CLOA),utilize the equivalent macro model of OA to replace OA; for the circuit after replacing, extract the unified fault feature of OA from NVIRF and establish fault dictionary to diagnose modular soft faults of the original circuit. Finally, experiments are demonstrated to prove the efficiency of the proposed approach.