The neutron flux monitor (NFM) is one of the most important diagnostic systems for ITER. Wide-range measuring algorithm (WRMA) is the core algorithm in the NFM system, which deals with the key task of neutron flux measurement. In this paper, the principle and implementation of WRMA, including counting and Campbelling algorithms, are introduced in detail, with error sources of the two algorithms analyzed. In order to study the performance of WRMA, we established a simulation system for neutron signal processing using MATLAB. According to the principle of neutron pulse distribution, the digital waveforms at different neutron flux levels were simulated as inputs to the WRMA module. The variation of measuring error was studied by comparing the counting and Campbelling results with actual input counting rate. In addition, the effects of different neutron pulse widths on the results of the algorithm were simulated. A preliminary experiment at HL-2A was carried out to validate the algorithm.