To study the mechanical properties of ferronickel slag–clay–cement (FNSCC) modified soil, the undrained triaxial compression tests of FNSCC modified soil under different ferronickel slag mixing ratios, moisture content and curing times were carried out. The stress-strain curve and secant elastic modulus of FNSCC modified soil samples under different working conditions were obtained, and the influence mechanism and failure mode of mechanical properties were analyzed. Based on the experimental results, the modified Duncan–Chang constitutive model suitable for FNSCC modified soil was established. The results show that the stress-strain curve of FNSCC modified soil has obvious strain softening characteristics. The peak strength first increases and then decreases with the increase of water content. The swelling failure of the sample with a water content of approximately 15 % is particularly obvious. The peak strength of FNSCC modified soil increases with the decrease of the ferronickel slag incorporation ratio. With the increase of the ferronickel slag incorporation ratio, the failure mode of specimens changed from bulging failure to shear failure. With the increase of curing time, the failure mode changes from a plastic failure such as bulging deformation to a brittle failure such as local tension crack. The secant elastic modulus of FNSCC modified soil decreases with the increase of axial strain. When the axial strain is less than 2%, the addition ratio of ferronickel slag, water content, and curing time have a great influence on the secant elastic modulus. When the axial strain is greater than 2%, each factor has little effect on the secant elastic modulus. The modified Duncan–Chang model can better reflect the strain softening phenomenon of FNSCC modified soil. The model parameters m, n and l are approximately linear with confining pressure. The model parameter m is most affected by the ratio of FNSCC modified soil, followed by the curing time, and the moisture content is the smallest. The curve calculated by the modified Duncan–Chang model is in good agreement with the measured curve, which verifies the rationality of the fitting parameters