Abstract: During the startup of pressurized water reactor (PWR) nuclear power plant, the secondary neutron source provides background count rate for the ex-core source range detectors, avoiding measurement blind zone and assuring the reactor’s safety. However, the usage of the secondary neutron source will bring biggish economic and environmental burdens and increase the plant operation risk from the secondary neutron source damage. So, the spontaneous fission neutron from irradiated fuel assembly can be used instead, which is called startup without neutron source. In this paper, the calculation method of detector count rate was studied and verified based on the measured data from the operation plant, and it provides a relatively perfect theoretical and methodological process for the theoretical prediction of the count rate of the source range detector. This method can provide theoretical support and reference for detector count rate analysis in demonstration of startup without neutron source, and optimization analysis of secondary neutron source loadage and disposal.