Abstract: The normal equation of linear regression is used to calculate the effects of high frequency components on the determination of secular trend of relative sea level(RSL).It shows that the periodic variation components(with period from several months to 18.61 years)have significant effects on the determination of secular trend of the short-time gaugerecords.The maximum flat filter which cut-off period is 4-year has been designed to filter out the high frequency components(with period less than 4 years)of monthly mean sea level.Secular trends of RSL are determ ined by the linear regression of low-passed gauge records.