Phenolic resin is considered to be the first commercialized synthetic resin and is widely used in industry.But a convenient and quantitative method for monitoring the structure of phenolic resin has seldom been proposed.In this paper，the expandable phenolic resin was synthesized by paraformaldehyde and phenol in the presence of aqueous NaOH.The gelation time and the CH2OH index measured by FT-IR spectrometer were employed respectively to investigate the reactivity of the synthesized expandable phenolic resins.The experimental results showed that the two analytical methods were very effective in tracing and differentiating the variation in reactivity of resins during the process of phenolic resin synthesis，and the analytical results agreed with each other.The effect of reaction time on the reactivity，viscosity and solid contents of the phenolic resins and further on the preparation of the expandable phenolic resins was investigated.