The soft release mechanism in a controlled hold-down and release system proposed recently was discussed in this paper and a finite element and rigid-body mixed model was put forward to analyze its release characteristic during launch vehicle liftoff. The rigid-body model was used to describe the movement of the launch vehicle and the finite element method was employed to analyze the finite deformation elastic-plastic contact of the soft release mechanism. The interface compatibility condition was enforced between the launch vehicle and the soft release mechanism. The mixed model was solved with an increment algorithm, which can be used to analyze the influences of the material property of the pin, the shape and size of the soft release mechanism, friction between the die and pin on the entire release time, the launch vehicle velocity and the consumed energy during liftoff.