The traditional iterative method is not applicable for some new reactors(such as supercritical water-cooled reactor) which have intense neutronic/thermohydraulic coupling(N-T coupling) behavior. The mathematical model of N-T coupling based on fixed point theory is established in this paper, with the calculation examples of SCWR, so as to present the convergence analysis of iterative methods in N-T coupling. The convergent criterion of N-T coupling is proposed, which can show the real-time convergence situation of iteration. Moreover, the self-adaptive relaxation factor and corresponding algorithm were introduced.