Abstract: Internal force distribution of concrete support has dynamic variation characteristics during tunnel construction. Researching the distribution characteristics and pre-detecting weak parts is one of the most important means to guarantee construction security. This paper considers the primary supports at the arc and the sidewall sections as the curved beam and the straight beam of the elastic foundation respectively, and theoretically deduces an inversion formula for the internal force of the primary support in a horseshoe tunnel based on the initial parameter method and measured pressure of the surrounding rock. The comparison between the measured and analyzed results shows that the above inverse analysis is practicable and can provide a theoretical basis for primary support design and construction technology optimization.