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When a very thin persulfate salt aqueous solution layer (mum) was sandwiched between two polymer films and strong UV light irradiated the assembly from the side transparent to UV light, a fast surface hydrophilic modification method for most of commercial polymeric materials was developed. For example, irradiating for 90 s and using 30 wt% ammonium persulfate, the static surface water contact angles of polymeric substrates decreased from 100 to 44degrees for LDPE, from 107 to 34degrees for HDPE, and from 73 to 15degrees for PET. The increases in surface hydrophilicity came from the formation of a sulfate salt group (SO4-NH4+)-ionized surface, which was characterized by XPS and ATR-FTIR. The surface topography of the modified polymer substrates were observed by SEM and AFM, and no visible etching effect to original surface were found. A possible reaction model named by ''confined photo-catalytic oxidation'' was put forward to interpret the above results.