Disclosed is a method for enabling flexibility of the exoskeletons or joints of aquatic organisms immersed in an ion liquid water solution to be maintained without destroying their original forms by reducing the difference in osmotic pressure between the inside and outside of the aquatic organisms, preventing dehydration of biological samples. First, the aquatic organism is put into a low-concentration ionic liquid to increase continuously the concentration of the ionic liquid, enabling the water content of the aquatic organism to be substituted by a high-concentration ionic liquid in their original forms. The use of the increasing method that gentle increase in temperature of the ionic liquid by means of natural seasoning reduces the osmotic pressure difference between the inside and outside of the aquatic organism, increasing the concentration of the ionic liquid inside of the aquatic organism.