Abstract: The on-line core monitoring system is an important operation supporting system for nuclear reactors of Generation Ⅲnuclear power plants. The SOMPAS（SNERDI On-line Core Monitoring, Prediction and Analysis System）, integrated with advanced three dimensional（3-D） neutronics and thermal hydraulics engine for on-line monitoring core condition, uses plant measurement data especially including in-core neutron detectors to monitor core power distribution and safety margin, which helps to improve the flexibility and to support the optimization of reactor operation. The development and verification of on-line monitoring function of the SOMPAS system are introduced. Numerical simulation and testing verify that SOMPAS monitoring function can provide reliable results, satisfying the expected requirements.