Janus transition metal dichalcogenides （TMDs） exhibit exceptional electronic， optical， and catalytic properties due to their unique asymmetric structure， showing broad application prospects in fields such as nanocatalysis and thermoelectrics. This study employs first-principles calculations to systematically investigate the stability and hydrogen evolution reaction （HER） performance of monolayer Janus MoSSe with typical vacancy defects. The results demonstrate that the Gibbs free energy of monolayer Janus MoSSe during the HER process is significantly reduced to approximately 0.5 eV， markedly lower than that of pristine MoSSe and conventional MoS₂ monolayers. Further research reveals that the introduction of external strain can effectively modulate the HER performance of defective Janus MoSSe. The performance enhancement is primarily attributed to the adaptive release of concentrated strain by dangling bonds in the defect regions， resulting in a notable tunable mode. This study elucidates the underlying mechanism of strain engineering in improving the HER performance of MoSSe， providing a theoretical foundation for the optimized design of efficient HER catalysts based on defective Janus TMDs.