Herein, we have developed a method for ultra-thinning MnO2 nanosheets by H2O swelling followed by a cetyltrimethylammonium bromide (CTAB)-intercalation exfoliation strategy. The thickness of the obtained U-MnO2 nanosheets was about 1-2 layers. As-prepared U-MnO2 exhibited higher turnover frequency (TOF) value of H-2 production from alkaline formaldehyde solution at room temperature as compared to that of pristine MnO2 (TOF per surface Mn atom: 2.7 vs. 1.1 h(-1)). With the ultra-thinning process, abundant surface oxygen vacancies (V-o) on U-MnO2 were demonstrated by X-ray photoelectron spectroscopy and extended X-ray absorption fine structure analysis, which could couple molecular O-2 and benefit for the breaking of C-H bonds from formaldehyde with the generation of (OOH)-O-center dot radical. Detailed reaction-pathway calculations showed that the O-2 assisted dehydrogenation of alkaline HCHO solution were thermodynamically favored on U-MnO2 with lower energy barrier of 0.32 eV as compared to that of 1.25 eV on pristine MnO2. This work not only provides an applicable method for synthesizing ultrathin MnO2 nanosheets but also gives an evidence towards more essential understanding of hydrogen evolution reaction from alkaline formaldehyde solution at atomic level.