In　this　work,　a　proton　exchange　membrane　unitized　regenerative　fuel　cell　with　a　25　cm(2)　active　area　and　a　transparent　window　was　designed　to　study　the　influence　of　mode　switching　from　the　fuel　cell　mode　to　the　electrolysis　cell　mode　on　the　cell　voltage　and　the　gas-liquid　two-phase　flow　behaviors　in　the　oxygen　flow　channels.　Results　indicate　that:　the　growth　rate　of　electrolysis　cell　voltage　before　the　water　pumped　to　the　oxygen　flow　channels　decreases　with　the　increase　of　the　fuel　cell　current　density;　while　the　growth　rate　of　electrolysis　cell　voltage　before　the　water　pumped　to　the　oxygen　flow　channels　increases　with　the　cell　temperature;　the　voltage　of　electrolysis　cell　mode　before　the　water　pumped　to　the　oxygen　flow　channels　decreases　with　the　increase　of　water　flow　rate;　the　different　voltage　reduction　speeds　are　attributed　to　the　different　water　flow　rates.　The　water　temperature　has　an　obscure　influence　on　the　cell　voltage　of　electrolysis　cell　mode　before　the　water　pumped　to　the　oxygen　flow　channels.