The invention discloses a boron concentration online monitoring system for a nuclear power station. The system is an online monitoring system for measuring the boron concentration in one loop of a reactor by adopting a direct neutron absorption method and mainly comprises a calibrating device, a detecting device, a measuring device, a remote display alarm box and a signal cable. The system omits a sampling vessel and a circuit pipeline, is completely independent of a sampling system of the nuclear power station and is directly arranged on a lower flow discharging pipeline, so that the safeties of equipment and operators are improved. The system has the advantages of simple structure, convenience for mounting, high response speed, high measuring accuracy, high digitalized degree and safety and convenience for use.