Abstract: The underwater sound absorbing coefficient of open-pore foamed aluminum with different pore structures were determined within a range of 3~24 kHz sound frequencies. The results show that all open-pore foamed aluminums with different pore structures have higher underwater sound absorbing coefficient,and that the underwater sound absorbing properties are attributed to their pore structures. When the pore size decreases, or the porosity and the thickness increase, the underwater sound absorbing properties will be improved.