Gas　diffusion　layer　porosity　can　affect　mass　and　charge　transfer　inside　proton　exchange　membrane　fuel　cells　and　influence　cell　performance.　Moreover,　the　electrochemical　reaction　rate　distribution　in　various　regions　inside　the　cell　is　not　uniform.　Proper　porosity　distribution　is　very　important　to　improve　cell　performance.　In　this　paper,　a　threedimensional　fuel　cell　model　with　three　steps　porosity　along　the　gas　flow　direction　is　established.　Four　cases　of　non　uniformly　distributed　porosity,　both　in　anode　and　cathode　gas　diffusion　layers　are　simulated,　which　are　compared　with　uniform　porosity　distribution　to　study　the　effect　of　these　structures　on　mass　transfer　inside　the　cell.　Then,　an　optimization　calculation　is　carried　out　to　obtain　the　optimal　porosity　distribution　along　gas　flow　direction　at　0.2　V　and　0.6　V.　The　numerical　results　indicate　that　non-uniformly　distributed　porosity　can　change　cell　performance,　and　porosity　increasing　along　the　gas　flow　direction　makes　cell　performance　be　better.　Porosity　increasing　along　the　gas　flow　direction　can　improve　the　uniformity　of　current　density　distribution　at　low　voltage.　Diffusive　mass　flux　plays　a　dominant　role　in　reactant　mass　transfer.　Higher　porosity　near　the　outlet　region　increases　total　mass　flux　at　the　interface,　and　the　proportion　of　diffusion　mass　flux　in　total　mass　flux　also　increases.