While　internal　electric　fields　alter　charge-separation　dynamics　in　solar-to-chemical　conversions,　a　greater　understanding　of　such　processes　is　necessary.　Here,　authors　analyze　charge　transfer　dynamics　modulated　by　built-in　electric　fields　and　identify　carrier　drift　distances　as　a　critical　parameter.　Construction　of　internal　electric　fields　(IEFs)　is　crucial　to　realize　efficient　charge　separation　for　charge-induced　redox　reactions,　such　as　water　splitting　and　CO2　reduction.　However,　a　quantitative　understanding　of　the　charge　transfer　dynamics　modulated　by　IEFs　remains　elusive.　Here,　electron　microscopy　study　unveils　that　the　non-equilibrium　photo-excited　electrons　are　collectively　steered　by　two　contiguous　IEFs　within　binary　(001)/(200)　facet　junctions　of　BiOBr　platelets,　and　they　exhibit　characteristic　Gaussian　distribution　profiles　on　reduction　facets　by　using　metal　co-catalysts　as　probes.　An　analytical　model　justifies　the　Gaussian　curve　and　allows　us　to　measure　the　diffusion　length　and　drift　distance　of　electrons.　The　charge　separation　efficiency,　as　well　as　photocatalytic　performances,　are　maximized　when　the　platelet　size　is　about　twice　the　drift　distance,　either　by　tailoring　particle　dimensions　or　tuning　IEF-dependent　drift　distances.　The　work　offers　great　flexibility　for　precisely　constructing　high-performance　particulate　photocatalysts　by　understanding　charge　transfer　dynamics.