In static identification based on measured deflection, given the current research typically employs the displacement influence matrix principle for linear fitting, this paper proposes a method using nonlinear fitting to identify prestress. Through finite element analysis and polynomial ridge regression, a correlation is established between long-term bridge deflection and prestress loss in bridges. Utilizing the measured bridge deflection, a hyperstatic equation system is constructed to solve for the residual prestress in operational bridges. An inversion validation was conducted on a prestressed concrete continuous rigid-frame box girder, showing that when prestress loss does not exceed 40%, the inversion error of this method is less than 3.53%, indicating high accuracy. This study provides a reference for the identification of prestress in similar types of bridges using measured deflection in the future.