In numerical and experimental analyses, we studied the effect of misaligning the cylindrical mirror rotation in shaping system on the far-field spot and beam quality of the laser during beam shaping based on Huygens--Fresnel diffraction theory. Increasing the rotation angle not only rotated and tilted the far-field spot of the shaping beam, but also elongated and defocused the spot. This aberration derives from the superposition of astigmatism and defocusing caused by rotating the cylindrical mirror. The results show that misaligned rotation also defocuses the shaping system with cylindrical mirror. We then quantitatively analyzed the correlation among the normalized phase coefficient and focal spot, in which the normalized phase coefficient is related with the center wavelength of the laser, size of the shaping beam, distance of the cylindrical mirrors and magnification factor. From the correlation coefficient, we can evaluate the sensitivity of the shaping system to the cylinder-mirror rotation disorder. Finally, we verified our calculated results using a corresponding experimental device. The experimental results agreed well with the calculated results.