As a basic component of software, the correct running of cyclic iteration program is of great significance. However, sometimes (e.g., when its NID is greater than 0) the rounding error (or representation error) in the calculation can lead to unstable results of the cyclic iteration. Based on the computing technology of “automatic dynamic adjustment of intermediate calculation accuracy”, a reliable calculation algorithm for cyclic iteration is presented in this paper. By using this algorithm, the value of arbitrary precision of cyclic iteration can be obtained. At present, the algorithm has been programmed and implemented in ISReal through C++ language.