Abstract: Abased tidal errors in TOPEX/POSEIDON(T/P) altimetric data were studied based on analyses of satellite orbit features and observations in China's near seas. The results show that in shallow waters strong tidal aliasirtg occurs in observations of T/P altimeter. Although the tidal correction provided in NASA's Geophysical Data Records works quite well for deep ocean basins, it can not effectively remove shallow water tides. However, since the main aliased tidal periods of T/P are shorter than 90 d, one can extract the longer seasonal signal of sea surface height through filtering procedure. The filtered results can then be used to study seasonal circulation(e. g.,the monsoon driven circulation in the South China Sea).Results of several filtering experiments using a FFT/IFFT filter show that T/P altimeter measurement is quite accurate. The deviations of filtered sea surface height consist very well with ground observation from tide gauges. The standard deviation between sea surface height observed by ascending and descendina orbits near a cross-over point is about 3 cm, which will satisfy demands of the South China Sea circulation studies.