The preparation of supported high-density metal nanoparticles (NPs) is of great importance to boost the performance in heterogeneous catalysis. Thermal transformation of metal-organic frameworks (MOFs) has been demonstrated as a promising route for the synthesis of supported metal NPs with high metal loadings, but it is challenge to achieve uniform metal dispersion. Here we report a strategy of “spatial isolation and dopant anchoring” to resist metal aggregation in the pyrolysis of MOFs through converting a bulk MOF into dual-heteroatom-containing flower-like MOF sheets (B/N-MOF-S). This approach can spatially isolate metal ions and increase the number of anchoring sites, thus efficiently building physical and/or chemical barriers to cooperatively prevent metal NPs from aggregation in the high-temperature transformation process. After thermolysis at 1,000 °C, the B/N-MOF-S affords B,N co-doped carbon-supported Co NPs (Co/BNC) with uniform dispersion and a high Co loading of 37.3 wt.%, while untreated bulk MOFs yield much larger sizes and uneven distribution of Co NPs. The as-obtained Co/BNC exhibits excellent electrocatalytic activities in both hydrogen evolution and hydrazine oxidation reactions, and only a voltage of 0.617 V at a high current density of 100 mA·cm⁻² is required when applied to a two-electrode overall hydrazine splitting electrolyzer.