This work focuses on the characterisation of damage mechanisms of GFRP-balsa sandwich under static 4-point bending by coupling Acoustic Emission (AE) monitoring and post-mortem microscope observations. A two-step clustering process coupling the three most relevant AE parameters (amplitude, peak frequency and duration) has been proposed and validated to better apply the K-means method to AE analysis of damage mechanisms of the complex GFRP-balsa sandwich. It verifies that cumulative counts can be the very meaningful AE indicator for the identification of initiation of microscopic and macroscopic damages in glass/epoxy composite skins. The importance of peak frequency and duration distributions has been emphasized to better identify balsa core damages and skin/core debonding. It is interesting to find that balsa wood can release signals with peak frequency higher than 200 kHz and amplitude lower than 60 dB even though there is no visible core damage by microscopic observations. Finally, it concludes that damage mechanisms of GFRP-balsa sandwich under 4-point bending could occur in balsa core: balsa/balsa interfacial debonding between small balsa blocks and microcracking in balsa core; at the interface between composite skin and balsa core; in the composite skin: matrix cracking, fiber/matrix debonding, delamination and fiber breakage. The correlations between these damage mechanisms and characteristics of the selected three AE parameters are also proposed.