Cold　start　is　one　of　the　urgent　problems　to　be　solved　in　the　process　of　commercialization　for　the　polymer　electrolyte　membrane　fuel　cell　(PEMFC).　In　this　study,　cold　start　processes　under　constant　power　start-up　are　simulated　with　a　three-dimensional　transient　model.　Evolutions　of　voltage　and　current　density　under　higher　and　lower　current　density　under　this　mode　are　elucidated.　The　water　accumulation　location　in　the　PEMFC　is　analyzed　and　effects　of　important　parameters　on　the　process　of　cold　start　are　quantified　under　constant　power　start-up　mode.　The　results　show　that　the　current　density　first　increases　and　then　decreases　when　the　PEMFC　starts　from　a　higher　current　density　under　constant　power　start-up　mode,　while　it　is　reversed　when　the　PEMFC　starts　from　a　lower　current　density.　Ice　first　appears　in　the　area　near　the　membrane　under　the　channel　and　the　hottest　area　in　the　cell　also　appears　below　the　channel.　At　the　lower　current　density,　the　distribution　of　water　in　ionomer　is　more　uniform.　Under　two　different　starting　current　densities,　the　changing　trends　of　the　conductivity　of　the　membrane　and　the　anode　catalyst　layer　are　different.　The　activational　heat　is　the　largest　heat　source　in　the　cold　start　process　under　constant　power　start-up　mode　and　the　utilization　of　ionomer　water　storage　is　higher　when　the　PEMFC　starts　from　a　lower　current　density.　The　temperature　of　the　inlet　gas　is　most　sensitive　to　the　cold　start　process　at　lower　current　density.　And　initial　content　of　membrane　water　and　membrane　thickness　have　important　impacts　on　the　cold　start　performance　when　PEMFC　starts　from　a　higher　current　density　due　to　the　redistribution　of　potential.