Effective　utilization　of　building　energy　flexibility　is　essential　for　balancing　the　power　grid　and　provides　economic　benefits　for　building　owners.　Active　thermal　energy　storage　systems　(ATES)　have　significant　load-shifting　potential　and　can　participate　more　directly　and　effectively　in　demand　response　(DR)　as　a　typical　flexibility　load.　Nevertheless,　there　is　a　lack　of　a　generalized　flexibility　potential　quantification　method　for　ATES　utilizing　phase　change　materials.　To　address　this　gap,　we　propose　a　generalized　ATES　flexibility　potential　quantification　method　that　incorporates　the　theoretical　heat　transfer　performance　model　of　ATES　and　flexibility　assessment　indicators,　and　conveniently　quantifies　the　flexibility　potential　of　ATES　using　easily　obtainable　parameters.　Validation　results　indicate　that　the　proposed　method　accurately　quantifies　the　flexibility　potential　of　ATES,　with　an　MAPE　consistently　below　10　%　and　an　average　MAPE　of　6.2　%　across　various　conditions.　The　flexibility　potential　quantification　results　demonstrate　that　ATES　exhibits　significant　flexibility　potential　for　grid-interactive　buildings,　achieving　a　maximum　flexibility　power　of　19.5　W/m2　and　a　flexibility　capacity　of　107.2　Wh/m2.　Additionally,　the　generalization　of　the　proposed　method　is　validated　using　actual　operational　data　from　existing　studies,　with　the　average　MAPE　across　various　working　conditions　and　parameter　settings　ranging　from　6.5　%　to　13.5　%.　These　findings　provide　valuable　guidance　for　selecting　ATES　equipment　and　formulating　DR　control　strategies.