Abstract: One software solution was presented to solve the unexpected reactor trip problem, which was caused by peak current generated during current range switch of intermediate range of nuclear instrument system (RPN). A lot of tests were performed in factory and on site. The results show that the peak current can be avoided effectively using this software solution, the current range can be switched smoothly and the system stability is improved significantly. Furthermore, this software solution was applied in many nuclear power plants, a stable output can be achieved by intermediate range at different core status, and consequently the economic loss caused by unexpected reactor trip can be avoided.