The commercial 42M Nd–Fe–B magnet was treated by grain boundary diffusion (GBD) with Pr₇₀Co₃₀ (PC), Pr₇₀Al₃₀ (PA) and Pr₇₀Co₁₅Al₁₅ (PCA) alloys, respectively. The mechanism of coercivity enhancement in the GBD magnets was investigated. The coercivity was enhanced from 1.63 T to 2.15 T in the PCA GBD magnet, higher than the 1.81 T of the PC GBD magnet and the 2.01 T of the PA GBD magnet. This indicates that the joint addition of Co and Al in the diffusion source can further improve the coercivity. Microstructural investigations show that the coercivity enhancement is mainly attributed to the exchange-decoupling of the GB phases. In the PCA GBD magnet, the wider thin GB phases can be formed and the thin GB phases can still be observed at the diffusion depth of 1500 μm due to the combined action of Co and Al. At the same time, the formation of the Pr-rich shell can also be observed, which is helpful for the coercivity enhancement.