The specimen is a reactor pressure vessel with a semi-elliptical surface crack with axial orientation and its 3D finite element numerical modeling is built. Based on J-A,2, two parameters method,analysis is performed by considering constraint effect,at T = 561 K,p = 10 MPa. Three different aspect ratio（ a/c） and Ramberg-Osgood exponent（ n） is analylized,namely a/c = 1/2,1/3,1/5（ a = 20 mm,c = 40 mm,60 mm,100 mm） and n = 10,15,20. The corresponding J-integral and constraint parameter A,2, is gained. The research results show that: J-integral increases with the decrease of aspect ratio a/c,and RambergOsgood exponent（ n） almost has no effect on J-integral. The increase of Ramberg-Osgood exponent（ n） will weaken the effect of aspect ratio a/c on A,2,. The most serious point is the deepest point of a crack.