Air-cooled low-temperature proton exchange membrane fuel cell stack applied with metallic bipolar plate is considered as a promising power source for an unmanned aerial vehicle. This paper presents a coupled electrochemical thermal model for simulating its dynamic characteristic. The impact of the applied metallic bipolar plate on the stack thermal balance is considered; an environmental model estimating atmospheric temperature and pressure variations with altitude is included. Our theoretical analysis shows that with altitude increased from 0m to 4000m, the output electric power declined rate is 4.7–6.5% at the current density of 400–800 mA·cm−2. To avoid severe stack degradation due to high stack operating temperature, minimum air stoichiometric ratio is required for maintaining stack thermal balance. When the altitude increases from 0 to 4000m, the minimum required air stoichiometric ratio decreases from 110 to 22 at the current density of 800 mA·cm−2.