Copper zinc tin sulfur selenide (Cu₂ZnSn(S,Se)₄, CZTSSe) thin-film solar cells have emerged as a promising photovoltaic technology due to their environmentally benign composition and abundant elemental constituents, though their current efficiency record remains constrained by substantial open-circuit voltage losses at the heterojunction interface. This review systematically examines the crucial role of heterojunction annealing processes in enhancing device performance, demonstrating that precisely optimized annealing parameters can effectively promote interfacial element redistribution, improve band alignment, and significantly suppress recombination losses. The low-temperature prolonged annealing approach has proven particularly effective in achieving superior interface passivation while maintaining structural integrity. Further interface optimization has been realized through innovative strategies including nanoscale interlayer engineering and cationic substitution approaches. By comprehensively analyzing recent advances in heterojunction annealing technology and highlighting promising research directions such as atomic-scale interface modification and computational optimization methods, this work provides valuable insights for overcoming the efficiency limitations of CZTSSe solar cells and advancing their commercial potential.