The　reverse　voltage　is　critical　for　the　performance　and　durability　of　a　proton　exchange　membrane　fuel　cell　(PEMFC).　The　reverse　process　will　cause　a　complex　kinetic　transformation　from　normal　to　abnormal,　leading　to　degradation　of　catalyst.　There　are　few　experimental　studies　and　some　numerical　simulations　reported　on　this　issue,　however,　the　process　of　reverse　voltage　under　dehydration　conditions　is　still　unclear.　This　article　explores　the　voltage　reversal　process　and　dehydration　phenomenon　of　a　PEM　fuel　cell　during　the　start-up　process.　The　effects　of　operating　parameters　on　the　occurrence　of　the　reverse　voltage　are　examined,　including　the　startup　current　density,　the　inlet　gas　relative　humidity　(RH)　and　the　gas　stoichiometric　ratio.　To　understand　the　reverse　process,　the　voltage,　high　frequency　resistance　(HFR)　and　the　outlet　gas　relative　humidity　during　startup　are　monitored　online　in　real　time.　Results　show　that　the　reverse　voltage　occurs　when　the　fuel　cell　starts　up　at　the　condition　of　low　relative　humidity,　high　current　density　load　and　high　hydrogen　stoichiometry　at　low　humidity.　From　the　beginning　of　the　startup　process,　the　humidity　of　the　anode　outlet　gas　decreased　significantly,　which　is　correlated　with　the　increase　of　HFR　and　the　drop　of　voltage.　By　time　constant　analyzing　of　the　process,　it　was　observed　that　the　dehydration　on　the　anode　side　of　the　membrane　electrode　during　the　startup　period　led　to　an　instantaneous　increase　of　HFR,　which　reveals　the　process　mechanism　of　voltage　reversal.　Finally,　we　suggest　a　startup　strategy　to　reduce　or　even　avoid　the　reverse　voltage.