Abstract: Based on the piecewise linear fault feature of the components in non-linear analog circuits,a unified fault diagnosis method for analog circuits is proposed based on iterative simulation before test.By dividing the parameter of linear component into sections before test,slope features of the simulation result points in the node-voltage plane are calculated after simulation in every section.For the section with slope change,iteration simulation is implemented after another parameter scanning,and then the points of smoothly continuous piecewise line locus are taken as fault diagnosis basis after test.For the diagnosis after test,the minimal distance from the test result point to each point set of piecewise line locus is calculated to locate the fault status of the circuit.Furthermore,the parameter section of the fault component can be identified according to the way of the parameter dividing sections before test.In this paper,we utilize the computer programming to implement the iteration simulation before test and fault diagnosis(fault location and parameter section identification)after test,which can diagnose hard faults(open and short faults)and soft faults(component parameters change to beyond the allowed tolerance range)of analog circuits with tolerance.Finally,simulation results and practical testing results are demonstrated to validate the efficiency of the proposed method.