The calculation method of stress field and birefringence at the fiber core center with any stress region shape is studied. The design of bow- tie fiber is optimized. With the help of solid mechanics module in COMSOL Multiphysics software, stress field distribution at fiber core center is studied when the stressinduced fiber has same stress area and different stress region shapes. The result shows that the numerical value acquired by integral method agrees with the value from software simulation. Therefore, the stress distribution and birefringence at fiber core center can be calculated by integral method directly even if the stress- induced fiber has any stress region shape. By means of stress element analysis based on integral method, it is found that the bow- tie fiber birefringence at fiber core is not the biggest compared with other type of stress- induced fiber if their stress area is the same. This is against the usual conclusion that the birefringence of bow- tie fiber is maximal. To get higher birefringence, bow- tie fiber is optimized again. As a result, a new fiber with“lunar”stress region is obtained.