Abstract: In order to study the effect of fly ash on the water content and dynamic mechanical properties of concrete under water confining pressure, different concrete test specimens (F00, F20, F40) with fly ash contents of 0%, 20% and 40% were made. The dynamic compression tests under water confining pressure (1, 3 and 5 MPa) and different strain rates (10⁻⁵, 10⁻⁴, 10⁻³ and 10⁻²/s) were conducted. Dynamic compression tests of dry concrete specimens in atmospheric environment were carried out as well. The effects of water confining pressure and fly ash content on concrete moisture content were analyzed, and the relationship between fly ash content, water confining pressure and strain rate and concrete dynamic strength was further studied in combination with water content data. The research results show that: ①Under the same fly ash content, the water content of concrete shows a trend of first increase, then decrease, and then increase with the increase of water confining pressure. Under the same water confining pressure, F00 concrete has the largest water content, followed by F40 concrete. F20 concrete is the smallest, indicating that the amount of fly ash has a significant effect on the water content of concrete. ② A cubic function is used to describe the relationship between the water content of different fly ash concretes and the water confining pressure. ③ Under different water confining pressure conditions, the increase value of concrete dynamic strength corresponding to the unit volume of water content of fly ash concrete is a quadratic function relationship with strain rate, and the relationship between dynamic strength increase value of fly ash concrete and strain rate under different water confining pressure conditions is established.