The Iterative Robin Boundary Condition(IRBC) has many advantages: the boundary can be set close to the scatterer；the internal resonance can be eliminated；maintaining the finite element system matrix symmetrical and sparse；the Robin Boundary needn’t to be set specially；it can be used in calculating the concave volume scattering. But when the IRBC is used to compute the 2-D electrically large targets，the iteration will not be convergent, and this can be settled by increasing the distance between the fictitious boundary and the scatter boundary. In this paper，a series of complex formulas of convergence coefficient about the distance and the scatterer radius are obtained by decomposing the incident wave into a series of cylindrical waves. Fitting the data obtained from the series of formulas，the succinct empirical formula is derived, which has provided a rule about how to set the distance between the fictitious boundary and the scatter boundary.