The crack tip mechanical characteristic and crack propagation rates are influenced by crack length, but current research of environmentally assisted cracking (EAC) focuses on long crack and small crack is usually ignored. The mechanical characteristic nearby small crack tip for single-edge crack panel specimens during the EAC in high temperature water was studied, the specimens was made of nickel-base alloy and the finite element method was adopted. The results show that the stress and strain is much higher for small crack than that of long crack, and this lead to the higher crack propagation tendency of small crack. A corrected method is introduced to calculate the plastic zone size of small crack. The Irwin correction method can improve the accuracy of J integral of long crack, but errors still exists for small crack. So the numerical method with elastic plastic fracture method is recommended to calculated J integral for small crack under larger load condition considering the lack of mature theoretical guidance about the small crack. The crack propagation process of the EAC of structure materials serviced in nuclear power plants is suggested to divide into two small crack propagation and long crack propagation considering to the anomalous mechanical behavior of the small crack.