Abstract: Pulsed neutron source method(PNS) is an important technique for the reactivity measurement of accelerator driven sub-critical systems. The CiADS sub-critical reactor model is established using Monte Carlo code to simulate the neutron transport process bombarded by pulsed proton beam, and the time spectrum of neutron flux is obtained. A code is developed by Python programming language to finish the pulsed neutron superposition process. It can obtain a stable delayed neutron background and the variation of neutron flux at different positions of the core under the continuously pulsed period proton injection. The reactivity measurement simulation of the sub-critical reactor by PNS method come true. Subsequently, the PNS method is used to simulate the reactivity of the CiADS core under different sub-criticalities and detection positions, and the results is compared with the reference values. It shows that this method can accurately predict the reactivity of sub-critical reactor at high effective multiplication factor ( k_\mathrmeff >0.94). The result nearby the spallation target shows a strong spatial effect. It needs to do experiments to figure it out.While, the outer fuel zone and the reflector assembly positions are more suitable for detector placement. Those conclusions can be used as a reference for the reactivity measurement of CiADS in the future.