Developing　oxide　supports　for　stabilizing　single-atom　catalysts　enables　more　flexibility　for　tuning　the　electronic　metal-support　interactions　(EMSIs)　toward　better　catalytic　activities.　However,　due　to　the　electronegativity　of　oxygen　anions,　single-metal　atoms　often　remain　positively　charged　in　these　oxide　supports　and　are　poor　at　binding　hydrogen　species　for　the　hydrogen　evolution　reaction　(HER).　Here,　we　report　a　ligand　charge　donation-acquisition　balance　strategy　via　an　amorphous　TiBxOy　support　to　tune　the　EMSIs,　which　lead　to　the　boosted　HER　mass　activity　of　a　single　Pt　atom　catalyst.　Based　on　spectroscopic　characterizations,　we　found　that　Pt　single　atoms　preferentially　bonded　with　nearly　neutral　B　atoms　originating　from　TiB2-like　species　in　the　Ti-B-O　framework　rather　than　O　anions.　Density　functional　theory　calculations　reveal　that　due　to　the　charge-transfer　balance　between　B-O　and　B-Pt,　the　nucleophilicity　of　Pt　was　tuned　to　an　optimum　state,　with　an　ideal　hydrogen　binding　energy　that　benefits　the　HER.　As　a　result,　this　Pt/TiBxOy　catalyst　achieves　a　high　HER　mass　activity　(37.8　A　mg(-1)　Pt)　and　a　turnover　frequency　(33.2　H-2　s(-1)　Pt　site(-1))　at　an　overpotential　of　50　mV　in　an　acid　medium,　outperforming　commercial　Pt/C　by　a　factor　of　34　and　33,　respectively.