Abstract: The central laboratory developed a special purpose Mass Spectrometer for UF6.When the instrument was used to determine samples of lower abundance( 235 U abundance less than 1%),the accuracy is poor (relative bias 1%) due to the secondary electrons and strong peak tails of the instrument.Based on the mass spectrometry theory and practical measurements,this paper analyses the origins of the two factors and their solutions.Through modification of the collector and the high vacuum system,the two factors have been eliminated.Index measurements have proved that the modification has produced the expected results (relative bias 0.5%) and it is successful.