The invention provides an ex-core nuclear detection system for a pressurized water reactor. The ex-core nuclear detection system is used for zero-power physics testing of reactors, and comprises an intermediate range detector and a reactivity meter, wherein the intermediate range detector is arranged outside a reactor pressure vessel and is used for measuring the nuclear power of the reactor; the reactivity meter calculates the reactivity of the reactor core according to data measured by the intermediate range detector; the reactive data can be used for measuring an isothermal temperature coefficient and a control rod value. Therefore, data measured by the intermediate range detector is transmitted into the reactivity meter, a test with high flux fluctuation can be performed, the gamma noise influence is small, the system can be far away from a Doppler heating point, the measurement result is accurate, low leakage and a low-leakage loading scheme can be implemented, the economic efficiency of fuel utilization is improved, and design of multi-reflection layer can be adopted, so that the economic efficiency and shock resistance of the fuel utilization are further improved, the measurement result is slightly influenced by spatial positions, and the influence of reactor cavity water injection transformation and the like because of RPN outward movement can be effectively reduced.