Abstract: The critical experimental device with spent fuel can be used to carry out critical experimental research on spent fuel storage. Considering its high radioactivity，the best way to carry out the experiment is to take the method of extrapolating water level to criticality. At present, the critical experimental device with spent fuel has not been built yet, thus the simulation experiment of water level extrapolation is carried out in a critical experimental device with similar core structure and physical characteristics, which is a uranium rod grid device with light water as moderator. In this experiment, the normal extrapolation curve cannot be obtained because of the shielding effect of water deepens with the rise of water level. In this paper, the extrapolation process was simulated by Monte Carlo program, and the characteristics of neutron count rate changing with water level were studied when the neutron source and detector were arranged in different relative positions, thus the optimal arrangement scheme of detector and neutron source was given. In addition, an extrapolation method with water level worth as extrapolation parameter was proposed, which eliminates the influence of uneven water level coefficient when extrapolating to criticality with water level, and makes the extrapolation result more accurate.