In this study, the uniaxial compressive mechanical properties of earth materials are tested, and the effects of four influencing factors, such as shape, size, curing age, and loading rate, on the strength, damage pattern, and stress-strain curve of the specimens are analyzed. The standard uniaxially compressed specimen size and the recommended loading rate are proposed for the earth specimens. The uniaxial compressive constitutive equations of earth materials are modified on the basis of the Illampas constitutive equation. By fitting the results of this study and typical literature tests, the applicability of the modified constitutive equation form to the uniaxial compressive test curves of soils in different regions of China based on standard sizes is verified. Finally, the formulae for calculating the parameters related to the constitutive equation of earth materials are established. In its application, only the compressive strength of 100-mm-cubic standard specimens with a curing age of 28 d needs to be measured to calculate and determine the specific values of the relevant parameters of the constitutive equation. This is a good reference value for promoting the development of computational analysis methods for earth structures and promoting the engineering design applications of earth structures.