Dropwise condensation has received significant attention due to its great potential to enhance heat transfer by the rapid droplet removal. In this work, droplet departure characteristics on a vertical surface, especially the droplet departure retention at low steam pressure and its effect on the heat transfer performance are investigated experimentally. The energy dissipation increases during droplet movement due to the increased viscosity at low pressure. Droplet oscillation caused by excess kinetic energy weakens and the dynamic contact angle (CA) hysteresis becomes apparent, which is not beneficial to droplet departure. Condensed droplets grow larger and fall more slowly at low pressure compared to that at atmospheric pressure. The droplet moves smoothly downward once it grows to departure size at atmospheric pressure while the droplet exhibits an intermittent motion at low pressure. Based on the droplet departure characteristics, a unified heat transfer model for dropwise condensation is developed by introducing the pressure-dependent departure velocity. The modified model very well predicts heat transfer performances at various pressures and the nonlinearity of heat flux varying with surface subcooling is quantitatively explained. This work provides insights into the heat transfer mechanism of dropwise condensation and offers a new avenue to further enhance heat transfer at low steam pressure.