Sessile evaporating droplets are widely encountered in various scientific and industrial fields, therefore, it is of vital importance for accurate prediction of actual droplet evaporation rates. For slow droplet evaporation, the heat and vapor transport of the droplet are diffusion-controlled, the classic isothermal model can predict the droplet evaporation well. However, for fast droplet evaporation, the convection flow, heat and vapor transfer are strongly coupled together, which brings great difficulty for accurate prediction. In this study, both theoretical analysis and numerical simulation are carried out to study the droplet evaporation with coupled flow, heat and vapor transfer fields. The droplet evaporation is investigated under different evaporative cooling numbers, droplet contact angles and substrate superheats. It is found that droplet evaporation can always be deteriorated by the evaporative cooling effect, while it can be enhanced by thermocapillary convection flow inside the droplet and natural convection outside the droplet. It is surprisingly found that the actual evaporation rates can be higher than those in the isothermal model, especially at low contact angle, high substrate superheat and weak evaporative cooling effect. Besides, a correction factor is proposed to quantify the deviation of actual evaporating rates in our fully coupled model from those in the isothermal model, and provides the dependence of the correction factor on evaporative cooling numbers, contact angles and substrate superheats, which can help predict the actual droplet evaporation rates. These findings may be helpful to get insight into the droplet evaporation and guide for its application.