The periodic lack of database about water level below low tide is a normal problem in the downstream area of a barrage in tidal estuaries,which may cause significant errors in harmonic analysis of the tidal level at that area.A modified method is proposed to solve this problem.The preliminary modification is to approximate the missing low water level by the neighboring measured high water level based on the tidal asymmetric characters,then the self-modification is repeatedly performed until the tidal harmonic constants keep unchanged.Verification results show the modification method can effectively and efficiently solve the question associated with the missing low tide data problem.This method is successfully applied in the forecast of tidal level in Gaobeipu,Ningbo in 2014.