[en] Highlights: • With sodium ions insertion, MoS₂ will first experience layer expansion and then phase transformation and finally fragmentation after triggering conversion reaction. • With sodium ions insertion, the HER activity of MoS₂ will gradually increase at the intercalation stage and further increase at the early stage of conversion reaction and finally slightly decrease at the end of discharging. • The possible structure-performance relationship is established. -- Abstract: Though enormous attention has been paid to MoS₂ in the field of electrocatalytic hydrogen evolution reaction (HER), the conventional 2H MoS₂ is hardly regarded as an alternative to noble Pt due to the limited number of active sites and semiconducting character. Herein, sodium ions were intentionally introduced into the MoS₂ to regulate its structure and thus HER activity. We found that the HER activity of MoS₂ would gradually increase at the intercalation stage and further increase at the early stage of conversion reaction. Based on various characterization techniques, the possible structure-performance relationship was established. The first increase in activity was ascribed to interlayer expansion and phase transformation, and the second increase in activity was mainly ascribed to increase of active sites due to the fragmentation of MoS₂ after triggering conversion reaction. Even though deep discharging MoS₂ to 0.01 V (vs. Na⁺/Na) would slightly impair HER activity, such an activity was still much better than that of pristine MoS₂, which might be originated from the doping of S into molybdenum oxide according to the DFT calculation.