With the development of the measuring instruments, many analytical techniques can be used to monitor entire process of a multicomponent reaction system.New monitoring data analysis methods represented by chemometrics have made remarkable progress, providing new ways to analyte chemical measurement data and various spectra for the optimization of chemical process and research of mechanism.A noval Alternation Least Square (ALS) based on fixed size bidirectional moving window evolving factor analysis (FSBMW-EFA) is proposed to determine the precise time of the transformation of the components during the reaction and to obtain the changes of concentrations as well as the spectra of the interested components in the mixture, achieving the goal of real-time monitoring.FSBMW-EFA is developed based on fixed-size moving window evolving factor analysis(FSMWEFA[1]).In FSBMW-EFA, the window of fixed-size moves successively along the time axis from two directions, forward and backward.Two tables of the eigenvalues against matrix centre, signed as forward-eig and backward-eig, is produced to determine the time accuately that resovles the problem of time shift in FSMWEFA.