The effect of cathodic hydrogen evolution on coercivity and thermal stability had been investigated in sintered NdFeB magnets. The magnetic properties, phase structure and morphology were systematically studied by SQUID-VSM, DSC, XRD, TEM and SEM. After cathodic hydrogen evolution, The Hcj decreased from 13 to 12.1 kOe. The temperature coefficients α declined from -0.253 %/℃ to -0.3229 %/℃, and the β declined from -0.7518 %/℃ to -0.7738 %/℃. A mechanism was proposed to explain the results. In the process of cathodic hydrogen evolution, parts of the generated hydrogen atoms reacted with the Nd2Fe14B phase and Nd-rich phase, formed Nd2Fe14BHx and NdHx, respectively. The formation of NdHx caused a volume expansion. Which resulted in intergranular cracks and stress. X-ray diffraction and morphology characterization confirmed the presence of these defects. These defects would significantly promote the nucleation of reverse magnetic domains, and further declined the coercivity and thermal stability of magnets.