Abstract: The comparative analyses are made between 65 seiche processes with the amplitude greater than 50 cm (the amplitude greater than 80 cm in Longkou Harbor) in main harbors along the coasts of the Bohai Sea and the Huanghai Sea and the surface weather charts.The weather situations accompanied with large amplitude seiche processes are also analyzed.The results show that:(1) Among the 22 cases of large-amplitude seiche,19 cases are related to thunderstorm weather(from which three cases are accompanied with squall lines,one case with tornado) and two cases are related to gale weather,and one case is caused by open sea thunderstorm.Thus,thunderstorm and squall line are marn factors for the formation of the large-amplitude seiche.Two weather types(the northern cyclonic,the cyclonic of the south of the Changjiang River,and southw est inverted trough) are benefit to produce thunderstorm and result in largeamplitude seiche.(2) The intensity of thunderstorm is stronger,the greater atmospheric perturbation and the larger amplitude of seiche.(3) The duration of seiche and main period of oscillation in each harbor are determined by the duration and propagation direction of the thunderstorm.(4) A transient variation of the pure wind speed and direction can also induce the large-amplitude seiche,but its amplitude is smaller than that produced by strong thunderstorm,which well combined with transient variation of atmospheric pressure,wind speed and direction.(5) The sea surface fluctuation triggered by the open sea thunderstorm can produce the long wave or isolated wave.When the wave propagates into harbor,it can also cause the large-amplitude seiche.