The tensile property and fracture appearance of U-2.5%Nb alloy (mass fraction) were studied under the conditions with relative humidity of 3.5%, 50%, 100%, underwater, hot water and water vapor, together with hydrogen pressure of 0.4, 1.0, and 2.0 MPa. The results show that when the strain rate is 3′10-5/s, with the environment humidity and hydrogen pressure increasing, the tensile strength of U-2.5%Nb alloy decreases, the proof strength non-proportional extension increases, and the plasticity decreases sharply. In the underwater environment, at the strain rate of 3′10-5/s the elongation after fracture decreases by about 84% and the reduction of area decreases by about 79% compared with those at the strain rate of 1′10-4/s. At the strain rate of 3′10-5/s and in the 2.0 MPa hydrogen pressure environment, the elongation after fracture decreases by about 20% and the reduction of area decreases by about 13% compared with those in the 0.4 MPa hydrogen pressure environment. The fracture surface of U-2.5%Nb alloy shows a ductile feature in the environment with 3.5% relative humidity. However, dimples of the alloy fracture surface reduce obviously and a brittle fracture appears in the underwater environment. The fracture surface of the alloy shows an evident cleavage and brittle feature in water vapor environment. The brittle feature of the alloy fracture surface is enhanced with the hydrogen pressure increasing.