The frontogenesis, atmospheric diabatic heating and net water vapor budget are analyzed to reveal the development and maintenance mechanism of the synoptic scale system of the Henan province extreme heavy rainfall from 19 July to 21 July in 2021 using automatic weather station observations and the fifth-generation European Center for Medium-Range Weather Forecasts atmospheric reanalysis data. Results show that Henan is located in the saddle area between the Northwest Pacific Subtropical High (NPSH) and the continental high. In this situation, the convergence in low levels is collocated with the divergence at high levels, which is conducive to the development and maintenance of the low-pressure system at 500 hPa during the extreme heavy rainfall. The frontogenesis mainly occurs in the lower troposphere and is consistent with the θ_se intensive region. The horizontal divergence term and horizontal deformation term play equally important leading roles in the frontogenesis. The horizontal distribution of the apparent heat source $⟨Q_1⟩$ and moisture sink $⟨Q_2⟩$ coincide with the area of heavy rainfall. However, vertical distributions show significant differences between Q₁ and Q₂. Q₁ features an obvious single peak structure with the heating center in the middle and upper troposphere, while the large value area of Q₂ is evenly distributed at 850—400 hPa. Q₁ is larger than Q₂ in the middle and upper troposphere. The potential temperature vertical transport term dominates in Q₁ and the specific humidity horizontal transport term dominates in Q₂, indicating that the release of regional-scale strong condensational latent heat has a positive feedback effect on the precipitation. With the continuous strengthening of the easterly air flow from the north side of typhoon In-Fa (2106), the strong wind shear and orographic uplift lead to an abnormally strong convergence of water vapor in the boundary layer. The net inflow of water vapor from the east and west dominates the total water vapor budget. The extremely strong water vapor transport in the boundary layer from the east plays a key role in maintaining and strengthening the extreme rainfall process.