In order to measure the on-orbit modulation transfer function (MTF) of medium resolution spectral imagers, an approach based on non-ideal step targets is proposed. By using targets with only one uniform side, the proposed method based on the step response is carried on the assumption that the line spread function (LSF) is symmetric. Compared with the traditional step edge method, this method achieves uniform results from the same target captured by a medium resolution spectral imager. The full width at half maximum (FWHM) derived by the proposed method differs from the traditional methods by only 0.03 pixel and the difference of the MTF value at the Nyquist frequency is only 0.02, which verifies the effectiveness of the proposed method. Since the using targets are very common in remote sensing images captured by medium resolution spectral imagers, the proposed method can be applied to more cases than the traditional approaches.