Crystal cascading is widely used for the compensation of walk-off angles in the nonlinear optical frequency conversion process. However, it is discovered that the conversion efficiency dramatically decreases when one of the cascaded crystals is rotated by 180° along the optical axis in the second harmonic generation experiment based on KTiOPO4 crystal cascading. The theoretical analysis indicates that the crystal rotation of 180° changes the sign of the effective nonlinear coefficient, which is equivalent to the generation of a phase mismatch of π and thus the second harmonic generation conversion efficiency decreases. A method based on air dispersion is proposed for the compensation of phase mismatch, which is characterized by simple configuration, solid temperature stability, low insert loss and low cost if compared with those for the compensation method based on wave plates. The experimental results show that the phase mismatch is compensated by air dispersion and the influence of crystal rotation on second harmonic generation conversion efficiency is eliminated. The validity of the theoretical analysis and the feasibility of the compensation method based on air dispersion are demonstrated.