Fatigue cracks generated during operation would alter the aero-engine blade vibration characteristics, manifested as frequency veering and mode shape switching. Study on the frequency veering can lead us to a further understanding of the blade crack evolution process. After building the FEM of aero-blade, the influence of crack parameters (crack location and length) on blade natural vibration and forced vibration characteristics was studied based on structural modal vibration theory, focusing on the variation laws of vibration frequency and mode shapes in the frequency veering area. The research shows that crack location and length variation would affect the blade frequencies and mode shapes, and thus lead to complex frequency veering, mode coupling and mode shape switching characteristics, which explains why mode shapes alter with the crack length and location in the same vibration mode.