The dynamic constitutive relationship of soil is essential for seismic dynamic calculations in geotechnical seismic engineering. Numerous experiments have demonstrated that cohesive soil exhibits cyclic strain softening characteristics under dynamic loading (equal-amplitude loading and unequal-amplitude loading) conditions. However, the Masing criteria (Masing's 2-fold method) cannot describe the cyclic strain softening behaviour of soil for equal-amplitude loading and often fail to estimate the damping ratio accurately (underestimation of the damping ratio at low strain; overestimation of the damping ratio at high strain). Furthermore, the Masing criteria are not applicable for nonequal-amplitude loading. To solve these problems, the research in this paper is based on the Davidenkov skeleton curve, and Masing's 2-fold method is utilized to establish a hysteretic curve. The cyclic strain softening coefficient and damping ratio adjusting coefficient were proposed for equal-amplitude loading to accurately capture the cyclic strain softening properties of cohesive soil. For nonequal-amplitude loading, Masing's n-fold method is employed to establish stress‒strain hysteresis curves, and using the customized constitutive interface reserved by FLAC3D, the development of this constitutive model is achieved on the Visual Studio (VS)2015 platform. The calculation accuracy of the model is verified by comparison with the theoretical values and experimental data, and the rationality of the constitutive model is verified by cyclic direct simple shear tests and cyclic torsional shear tests of soil subjected to equal-amplitude loading and centrifuge model tests of ground bearing unequal-amplitude loading, which provides effective help for the study of earthquake disasters in cohesive soil sites.