Abstract: The reactivity meter has to be calibrated every time before measuring reactivity by using period method during PWR physical test. Meanwhile, the period method is not accurate enough when the large negative reactivity is introduced due to the effect of the transient items. The numerical calculation analysis was applied to solve point-reactor neutron kinetics equations for more accurate results. Therefore, the coefficient K method was discovered by comparing the test result of neutron density with the calculation result of period method. The coefficient K method can simplify the way dealing with the transient items and make period method still available to the calibration of negative reactivity.