In order to study the influence of the upper beam structure stiffness change on the car body modal, the detailed finite element model (FEM) of stainless steel passenger car body structure was established. The upper beam was taken as the research object, and the unit mass stiffness change was achieved by changing its unit mass inertia moment. Results showed that: the first-order vertical bending frequency initially increases with the upper beam structure stiffness by logarithmic form, and then enters the gentle stage to remain unchanged, finally increases linearly; the first-order transverse bending frequency initially increases with the upper beam structure stiffness by logarithmic form, and then increases linearly. This study could improve vehicle modal performance, effectively avoiding the external excitation frequency to prevent resonance.