The invention discloses an improved causeway water overlaying preloading method which comprises the steps as follows: 1), the periphery of a construction region is manufactured into a causeway to a required water overlaying height by using a water bag or other water containers; 2), a vacuum film is laid above the construction region; 3), pumping equipment is started, pumped water is injected into the vacuum film in the causeway, and water loads are added to the construction region; and 4), when a consolidation degree of a to-be-treated soil body meets a design requirement, construction is stopped, and water in the causeway is drained. According to the method, a large amount of soil is not required to be carried for manufacturing the causeway, and only local materials are used, so that cost is reduced remarkably compared with a conventional vacuum water overlaying preloading method; and the weight of the water is larger than that of the soil, the water is taken as the loads, so that the effect is better, the construction period can be shortened, and the method has considerable economic benefits.