The existing multiscale principal component analysis method provides an effective way to monitor industrial processes with multiscale features due to the influence of events at different time-frequency values

but there are two main shortcomings with respect to this method：a reconstruction step is needed which results in a great number of monitoring models to be constructed; and Haar wavelet is used to do a wavelet transform for multiscale analysis

but it is not continuous and therefore not good at approximating practical signal.Hence

a modified multiscale monitoring method was proposed，which not only eliminated the reconstruction step based on the analysis of scale feature of the fault but also replaced Haar wavelet with sym wavelet

which was continuous and had a higher order vanishing moment and thus was more suitable to extract multiscale features.In particular

the proposed method gave an alternative multiscale way to determine the location and duration of two typical faults : step fault and oscillating fault

and solve the problem of boundary effect and signal alignment accompanying the introduction of sym wavelet.Finally

the effectiveness of the proposed methods was verified by simulated experiments on a standard CSTR problem.