Slit spatial filtering technology can be used to significantly ease or even solve problems related to plasma pinhole closure and large scale in current high-power lasers for fusion, which has broad application prospects in fusion laser facilities. According to the diffraction theory of aberrations, this paper numerically analyzes the aberration effect on efficiency of a three-lens slit spatial filter. Two important parameters (near-field contrast of output beam and intensity on inner edge of filter slit) are considered as evaluation parameters of aberration effect in a given filtering system, and the permitted percentages to be changed are given. Different types of aberrations with different magnitudes are introduced into the filtering system, and the tolerance of each aberration is obtained according to the change of parameters. The introduced filtering system is compared with conventional pinhole filters. The results demonstrate that all types of aberrations on the lens may introduce various inhomogeneous phase changes in the wave-front of a light beam and degrade beam quality. In addition, dispersion or deformation caused by aberrations may change the intensity irradiated on the inner edge of slit apertures, which is reduced continuously with increased aberrations. The most noteworthy aberrations relative to the three-lens slit spatial filter are the spherical aberration and field curvature of the first cylindrical mirror.