The invention relates to an online soft measurement technique, in particular to an online soft measurement method for the activity of thioglycolic modified ion exchange resin catalyst in the ion exchange resin bisphenol A condensation reaction, which utilizes the in-situ data of bisphenol A condensation reaction to realize mathematical regression and support vector machine nonlinear regressive analysis, to obtain a soft measurement hybrid model among four operating variables and the catalyst activity of the reaction system, such as alpha=f(t)+g(flow, rate, T), wherein alpha presents catalyst activity, t is the accumulated usage time of catalyst, f(t) is a mathematical expression for the accumulated activity monotonic reduction of the catalyst along time, flow, T and rate respectively present three key factors affecting the catalyst activity such as: reactant flow (load), phenol/acetone flow ratio (hydroxytropone ratio), and reaction temperature, g(flow, rate, T) presents the effect relationship of the three factors on the catalyst activity. In actual application, the four operating variables are input into the model to calculate the activity of the ion exchange resin catalyst in bisphenol A production.