Abstract: To determine the optimum water content of light weight soil with different mixture proportions, three kinds of models for the optimum water content are put forward: ρ_d–w model, qu–w model and RS-w model. It is supposed that three models can be expressed as a uniform downward opening parabola function. Analytic expressions for the optimum water content are theoretically derived. To test the above three models and to determine model coefficients, physical mechanical properties of light weight soil are studied by laboratory tests. The results show that the dry density, unconfined compressive strength and specific strength first increase and then decrease with the increase of water content, each of them has an optimum water content, and their curve shapes are similar to parabola. The measured data are analyzed based on the three models, the optimum water content calculated by models are very close to that obtained directly by the graphic method. By use of the precursors’ research achievements, the corresponding optimum water contents of qu–w model and RS-w model are almost the same, but they are slightly different from the corresponding optimum water content of ρ_d–w model. The optimum water contents of the three models are essentially the same. The optimum water content of light weight soil is the sum of physical and chemical water demand, it is an external appearance of water demand of complex physical and chemical reactions, and it is a theoretical basis for the three models to be unified. Taking RS–w model for an example, the water demand rate of mass of different components are calculated, and the results are closely related to properties of materials.