This work focuses on moisture effects on mechanical properties of GFRP-balsa sandwich structure experimentally and numerically. Firstly, relationships between Moisture Content (MC) and Acoustic Emission (AE) wave velocity have been demonstrated by Hsu-Nielsen tests during the water immersion tests. And then, a new methodology was proposed to predict the elastic modulus of skin of wet sandwich structure based on the relationship between MC, AE wave velocity and skin elastic modulus. Static 4-point bending tests were monitored by InfraRed Thermography (IRT) to verify the good consistency between bending stiffness reduction and elastic modulus prediction of wet sandwich by the new proposed method. Furthermore, Progressive Damage Analysis (PDA) model has been developed by introducing the modified elastic modulus, strength, as well as fracture energy of the wet skin, to quickly predict the bending stiffness, fracture load, displacement and damage evolution process of wet GFRP-balsa sandwich. Finally, it demonstrates that the damage localization by PDA model has a good concordance with the observations by IRT technique.