Abstract: Based on the neutron multiplication theory, the physical start-up model is established. The processes of lifting control rod discontinuously are calculated by this model, and the results are compared with the analytic solution of the point-reactor kinetics. Based on this, the effects of sub-criticality, lifting control rod durations and speeds on the physical start-up are investigated, and it shows that the sub-criticality is an importance parameter in the physical start-up. The conclusions are very important to the safety analysis and operation of the ship nuclear reactors.