Abstract: A method based on Sagnac interferometer for UV ultrashort pulse amplification is introduced．It provides automatic and phaselocked recombination of multiplexed beams in principle．The effect of optical components' adjustment tolerance on the interferometer is analyzed．And a practical multiplexing scheme use polarization sensitive reflector and it's experimental result is presented．A pulse of 2~3mJ,50mm in diameter is sent to the amplifier and abtained a stable output of 100mg, with a maximum of 120mg.