Abstract: In order to make up for the important theoretical defect of the lack of continuity assumption in Soil Mechanics, the continuity assumption in Soil mechanics was properly put forward on the basis of systematically combing and studying the concept of continuity in multiple disciplines, and then the concept, category, attribute and modeling idea of soil constitutive model were clarified. The research results show that the continuity assumptions of Continuum Mechanics and Dynamics of Fluids in Porous Media provide useful references for the continuity assumptions of Soil Mechanics, but both have certain defects. The viewpoint and method of Theory of Mixtures are suitable for describing the continuity of soil, and can avoid the difficulties encountered in defining porosity; Continuity is a relative concept. When the external feature size is much larger than the internal feature size of the medium, the research object can be regarded as a continuous medium. The existing constitutive models of soil are all based on continuum mechanics and generally belong to phenomenological models. In this paper, the concept, types and modeling ideas of constitutive equation are clarified, that is, constitutive equation is a mathematical model of macro properties of substances. The constitutive model of soil includes various aspects such as water retention, water permeability, gas permeability, heat transfer, yield, deformation, strength, meso-structures evolution, thermodynamic effects, phase transformation laws, etc., but it is not limited to stress-strain relationships. The study of soil constitutive equation should focus on specific soil types and grasp the main influencing factors; When studying the mechanical properties and constitutive models of special soils such as undisturbed expansive soil and undisturbed loess, the influence of their meso-structures and damage evolution should be considered. When studying the water holding properties, mechanical properties, thermodynamic properties, and constitutive models of bentonite, saline soil, and contaminated soil, the influence of physicochemical interactions between soil particles should be considered.