Abstract: Based on the fortnight measured tidal elevation in June-July 2005 in the Oujiang estuary, the tidal asymmetry, and its time-space variation are studied by three methods. The research results show that the traditional method of determining tidal asymmetry just by the relative amplitude of M4 and M2 is not perfect in theory. The relative amplitude and the relative phase as well as other constituents would affect tidal asymmetry in combination. Tides in the Oujiang estuary are the semidiurnal tides. M4 plays the most significant role in the tidal asymmetry. Longitudinal variation is more obvious than the lateral one. The tide distortion becomes greater from the river mouth towards upstream. It is relative weak and has no marked tendency to variation outside the month due to large spatial scale and water depth. Temporally, the tidal asymmetry is considerable during spring and neap tides and more remarkable during the spring tides than neap tides because of the Msf. And as it may be seen from the analysis above the tidal asymmetry variation during the spring and neap tids must be caused by the constituents of the Msf.