The invention provides a radial arrangement method of fixed detectors for a nuclear reactor core loading 157 fuel assemblies. The method comprises the following steps: determining the fuel assemblies equipped with control rod groups to be fuel assemblies not equipped with the fixed detectors; arranging the fixed detectors from the second row in an interlacing manner; adding the fixed detectors at proper positions or locally adjusting the positions of the fixed detectors; actually measuring state parameter values of the fuel assemblies equipped with the fixed detectors; carrying out interpolating to obtain the state parameter values of all the fuel assemblies in the nuclear reactor core; determining an optimal solution from the fourth series of solutions to be selected by using a statistical tool. The invention further provides the nuclear reactor core which comprises the fixed detectors and loads the 157 fuel assemblies. The radial arrangement method guarantees the precision and the speed of calculation of an on-line reactor core monitoring system, and further improves the economy of the on-line reactor core monitoring system.