Abstract: The sea level data observed by tide gauge stations are important basic observation information for monitoring sea level change and determining mean sea level. This paper analyzes the difference in relative sea level trends of tide gauges in different observation periods by sliding period method and time period accumulation method, discusses the influence of time period selection on the calculation of relative sea level change, and uses data of tide gauge stations on the east coast of the United States to supplement and verify the analysis results. The experimental results of tide gauges in China show that the maximum difference of relative sea level change rate in different observation periods is 27.5 mm/a, and the maximum difference of annual sliding results in adjacent years is 7.1 mm/a.With the accumulation of years in the observation period, the relative sea level change rate tends to increase at a constant speed, and in order to obtain the relative sea level change rate with the MSE (mean square error) of 0.5 mm/a, the observation years required by the tide gauges are 22-28 years, and the determination of the relative sea level trends with the MSE of 0.2 mm/a requires 40-54 years, which is consistent with the supplementary experimental results of tide gauge stations in USA.