The ex-core nuclear instrumentation system is a critical safety system in pressurized water reactor (PWR) nuclear power plants, responsible for monitoring the neutron flux outside the reactor core. Its core function relies on the scientific selection of detectors. As a representative model of China’s independently developed third-generation nuclear power technology, the HPR1000 places high demands on the reliability and advancement of its nuclear instrumentation system. This paper systematically compares the technical principles and engineering applications of three types of detectors—source range, intermediate range, and power range—and evaluates the performance characteristics and applicable scenarios of mainstream technologies such as boron-coated proportional counters, gamma-compensated ionization chambers, fission ionization chambers, and uncompensated ionization chambers. Based on in-depth analysis, the paper proposes a detector configuration logic for HPR1000, in which boron-coated proportional counters, fission chambers, and uncompensated ionization chambers are respectively selected for the source, intermediate, and power ranges. This configuration balances measurement accuracy, gamma interference resistance, and reliability under accident conditions. The paper also explores future trends in ex-core nuclear instrumentation systems, including wide-range detector technology, intelligent signal processing, material innovations, and domestic manufacturing, providing theoretical support and technical guidance for enhancing the intrinsic safety of nuclear instrumentation systems.