Stress relaxation is a significant characteristic of titanium alloys at elevated temperature and load, which is the theoretical foundation of hot sizing and heat treatment. The tensile stress relaxation behaviors of a Ti-6Al-4V sheet over the medium temperature range from 923 K to 1023 K and at several strain levels were investigated. Comprehensive analysis results indicate that the stress relaxation rate increases with the increase of temperature. The residual stress in Ti-6Al-4V alloy reaches the relaxation limit gradually after a period of relaxation. In addition, the stress relaxation limits reach the same value for the different initial stresses at the same temperature. Furthermore, an explicit constitutive equation with a cubic delay function was established and its prediction precision is as high as 97% and this has laid the foundation of process design and theoretical analysis. Finally, the implicit creep-type constitutive equation was developed to describe the stress relaxation behavior of Ti-6Al-4V alloy, and identified material parameters were input into ABAQUS to simulate a stress relaxation process of Ti-6Al-4V alloy due to the creep forming. Comparison results show that the stress change simulated is in agreement with the stress relaxation curve. It is proved that the creep-type constitutive equation is valid for the simulation of stress relaxation.