Abstract: The mathematical model for stress-strain curve is an important issue to study the mechanical behaviors of seashore soft soil. Based on the properties of developing process, the primary mathematical behavior that stress-strain model should fulfill is analyzed. Mathematical investigation on two conventional stress-strain models, namely hyperbolic model and exponential model, is conducted. Difference between the conventional models is remarked based on the Taylor series expansion, and stress index is proposed and employed to signalize their shortcomings. A composite exponential-hyperbolic stress-strain model with three parameters is established, and a formula for the improved tangent modulus is deduced. The mathematical analysis shows that model functions and stress indexes of the conventional models are simplified expressions of those of the new three-parameter model, respectively. The proposed model absolutely overcomes the shortcomings of the conventional models. According to the test data, an approach to determine the model parameters is presented. Finally, good accuracy of the new model is proved.