The present invention discloses a method for characterizing a membrane in a wet condition using a positron annihilation spectrometer and a sample holder thereof. Positron annihilation lifetime spectroscopy (PALS) has been know to be an invaluable tool for investigating local free-volume hole properties in various materials. Accompanying with the method and sample holder disclosed by the invention, PAS and PALS can measure the properties of various materials, such as free volume and layer structures both in the dry and wet states.