Abstract: To clarify how assembly constraint methods affect the modal characteristics of centrifugal impellers, this paper takes a centrifugal impeller as the research object and systematically compares the natural frequencies and mode shape characteristics of the impeller with different vibratory nodal diameters under two different constraint conditions. Combined with optical fiber blade tip amplitude measurement tests, the study verifies the accuracy of the simulation results. The results indicate that the third-order nodal diameter vibration is insensitive to the constraint method, with highly consistent calculation outcomes under different constraints. In contrast, the first-order nodal diameter vibration is significantly affected by the constraint method; only some modes exhibit correspondence between the two constraint schemes. Among them, Constraint Method A aligns better with the actual operating conditions, yielding resonant speeds consistent with experimental measurements. Mechanism analysis reveals that low-order nodal diameter modes involve global cooperative motion, where boundary conditions significantly alter the system’s overall stiffness matrix. Conversely, high-order nodal diameter modes exhibit localization characteristics, primarily depending on the blades’ inherent stiffness, and are insensitive to remote boundary conditions.