The far-field intensity distribution of 6 circular arranged coherent fiber lasers is simulated numerically and its power convergence is estimated based on Fraunhofer diffraction theory. The results show that the intensity distribution at far-field is equivalent to the product of a Gaussian envelop and the interference distribution of several planar waves. When the diameter of the arranging circle is less than 5 times of the radius of a unit laser, over 90% of the power will drop into the 86.5% power circle of the unit laser. The calculation leads to the conclusion that power convergence of the combined beam can be optimized by adjusting the arrangement, where the convergence can even exceed that of a single unit laser beam.