In this article,three dimensional elastic-plastic curved crack tip plasticity area under quasi-static loads has been mainly studied,and three dimensional curved crack tip plasticity area has been calculated as a practical application of a second order perturbation method,where the effects of quasi-static applied stresses and quasi-static normal and shear stresses on the boundaries of plasticity area are synthetically taken into considerations. Projected length of three dimensional curved elasticplastic crack tip plasticity area on straight crack extension line portion has been calculated by numerical solutions,diagrams have been constructed to analyse the transformation relationships between curved crack tip plasticity area size and three dimensional crack body geometrical measurement. Curved crack tip plasticity area size will decrease when three dimensional crack body thickness increases. When crack body thickness increases evenly,three dimensional curved crack tip plasticity area size will continuously decrease,furthermore,the decrease extent will be more and more slightly,and tend to plane strain state ultimately.Curved crack tip plasticity area size will increase when external loads increase with the same crack body thickness. A new theoretical model where curved crack tip plasticity area size was calculated has been established in three dimensional crack body.