Abstract: The confirmation of reactor neutron source is very important to the safety during reactor start-up, critical experiment and reactor start-up without neutron source. By analyzing origins of neutron sources in light water reactors after shutting down, it was made sure that the neutron sources in reactor core were mainly composed by the following four parts: spontaneous fission neutron source, (α, n) neutron source, primary neutron source and secondary neutron source. The calculation methods and principles of these four kinds of neutron sources were studied. A calculation model was built, and in which simplification measures were deduced to improve calculation efficiency. Based on this, a computer code was developed to calculate the intensity of neutron source at any specified time after the reactor was shut down. Comparing with the reference examples, the calculation results of the code coincide with the examples with a deviation no more than 4%. This neutron source calculation code has been validated and delivered to users.