In order to study the model simplification of stiffened plate structure in low frequency vibration, based on the small deflection thin plate bending theory, through mass equivalent and stiffness equivalent, the density formulation and the bending stiffness formulation of stiffened plate equivalent to orthotropic plate are obtained in this paper. Taking the unidirectional stiffened rectangular plate as the research object, the validity of the equivalent method is proved by calculating the error of the finite element solution of the natural frequency of the stiffened plate and the analytical solution of the natural frequency of the equivalent orthotropic plate. The influence of mass equivalence, stiffness equivalence and structural parameters on the accuracy of the equivalent method is discussed. The results show that stiffness equivalence is the main factor affecting the accuracy of the equivalent method, and the effect of quality equivalence can be ignored. The height of beam section, the number of beams and the thickness of plate all have a certain impact on the accuracy of the equivalent method. Finally, this paper makes a quantitative comparison and analysis of the three structural parameters by defining dimensionless parameters. The results show that the number of beams is the most significant structural parameter that affects the accuracy of the equivalent method, and the additional bending stiffness ratio within the quadratic order of 10 can ensure the accuracy of the equivalent method.