Wood has been widely used as a building and interior furniture decoration material due to its excellent insulation of heat and electricity, strong buffer effect on vibration, high strength, etc. Generally, wood materials should be processed by pyrolysis technique in order to improve corrosion resistance, flame retardancy, and dimensional stability. However, during pyrolysis, the main components of wood, such as cellulose, hemicellulose, and lignin, undergo chemical reactions causing the damage of the internal structure. With existing non-destructive evaluation methods, such as thermography and radiography, it is difficult to detect the degree of delamination within the wood due to its anisotropy physical properties and complex internal structure. In this work, a novel normalized time-domain integration method is proposed to detect the delamination of wood materials at different pyrolysis temperatures. The experimental results show the great robustness and detection efficiency of this method. Furthermore, numerical simulation and experiments are used to build the relationship between the terahertz time-domain signal and water content. Compared with spruce and oak, meranti is more resistant to pyrolysis and has higher structural stability. Display omitted •The variations of several parameters are discussed at different pyrolysis T °C.•The amplitude at a specific frequency is used to characterize the water content in the woods.•Numerical simulation and experiments validated the feasibility of the proposed new evaluation technique.•A novel normalized time-domain integration (NTDI) method is proposed for the inspection of internal delamination.•All the woods are detected using NTDI method and X-ray photography.