In order to ascertain the bound water of high liquid limit soil and its proper testing method, this research provides a basis for further research on compaction mechanism of high liquid limit soil, selection of high liquid limit soil in Hainan Wan Yang Expressway, unit-weight method and isothermal adsorption method were used to study the bound water test. The results show that the unit-weight method test results are greater than the isothermal adsorption test results. The bound water content measured by two methods increases with the decrease of particle size, fine particles＞ original soil＞ coarse particles. The strong binding water content of the fine-grained soil samples measured by the unit-weight method was 2.57 times that of the coarse-grained soil, and the strong binding water content of the fine-grained soil samples measured by the isothermal adsorption method was 2.12 times that of the coarse-grained soil. The hydration specific gravity of the high liquid limit soil and the density of the bound water all decrease with the increase of the adsorption bound water. Adsorption of water from water vapor by isothermal adsorption, because clay is stacked and has aggregates, water molecules are only adsorbed on the surface of the clay, so the test results must be small, the process of adsorption and binding of water from liquid water by unit-weight method is more consistent with the actual situation of raw soil. It is recommended to test the bound water content of high liquid limit soil by unit-weight method.