Objective To investigate the effects of high-definition transcranial direct current stimulation (HD-tDCS) on the modulation of the H-reflex and M-wave during ankle dorsiflexion-plantar flexion fatigue tasks to provide direction for the application of HD-tDCS in mitigating neuromuscular fatigue. Methods Twenty healthy young male participants were recruited and randomly assigned to either the real stimulation or sham stimulation group, with 10 participants in each group. The intervention consisted of a 5-day single-blind HD-tDCS application (duration: 20 min; intensity: 2 mA; target: Cz). Baseline measurements of the H-reflex and M-wave under resting conditions, M-wave during maximal voluntary isometric contraction (MVIC) of the dorsiflexor muscle, and MVIC torque of the dorsiflexor and plantar flexor muscles were obtained. An ankle dorsiflexion fatigue task was performed to determine the time to achieve fatigue for the task. The same fatigue task was repeated and evaluated one day after the intervention. A repeated-measures two-factor (stimulation condition × pre/post fatigue) analysis of variance (ANOVA) was used to analyze the effects of independent variables on the mechanical properties of the muscles and α-motoneuron conduction characteristics. Results After fatigue, voluntary activation (VA), maximal H-reflex (Hmax), maximal M-wave (Mmax), and dorsiflexor and plantar flexor MVIC torques in both groups were significantly reduced compared with pre-fatigue levels (P<0.05). However, compared to the real stimulation group, the sham stimulation group showed a more significant decline in VA and plantar flexor MVIC torque (P<0.05). Conclusions A continuous 5-day HD-tDCS intervention can effectively increase α-motoneuron activity at the spinal segment. It can also exert an inhibitory effect on reducing information transmission capacity at the peripheral neuromuscular junction under the ankle dorsi-plantarflexion fatigue task.