In present work, the influence of the oxygen content variation on the Dy increment and the coercivity increment in Dy grain boundary diffusion processed (GBDP) Nd-Fe-B sintered magnet was investigated. Various magnets including both high and low oxygen types were processed and the latter one showed higher increments either in Dy contents or in Hcj. Nine specimens with 0wt. % original Dy contents were compared after GBDP, and it was proved that decrease of oxygen is beneficial to Dy content and Hcj increase. Furthermore, Dy content gradient in each GBDP sample showed that the gap between the outer and interior region in low oxygen magnets were narrower. According to the EPMA figures, Dy-rich shells were more continuous and brighter and almost completely surrounded each matrix grains in low oxygen GBDP magnet. The optimization of the microstructure enabled the low oxygen GBDP sample to gain a higher anisotropic field increment. Decrease of oxygen lead to Nd-rich phase surround the matrix phase more homogeneously and continuously, providing a more continuous tunnel for Dy to diffuse into the magnet, ultimately inducing a further enhancements of Dy content and Hcj.