We propose an offaxis parabolic mirror (OAPM) afocal scanning system which can improve the image quality in the marginal area of twophoton microscopy. Compared with the traditional double lenses afocal scanning system, the OAPM afocal scanning system consists of a pair of parabolic mirrors, and a galvo scanner is located at one of the foci of the parabolic mirror. Both the simulation and experiment results show that the OAPM afocal scanning system can significantly reduce the astigmatism, the field curvature and the distortion introduced on the edge of field of view. Furthermore, adopting the proposed afocal scanning system, we build a twophoton microscope with a field of view of 2.4 mm×2.4 mm and a lateral resolution of 1 μm. Using this system, the neuron axons in the mouse brain slice can be clearly resolved.