Filter multi-pass test can more accurately evaluate the performance of filter.Filter multi-pass test bench includes contaminant injection system,filter test system and online particle counting system.Based on ISO 19438-2003 standard and ISO 4548-12∶2000 standard,the contaminant injection system was studied,an improved solution was put forward based on experiments.The improved contaminant injection system can effectively improve the stability of the inject flow rate and pollution uniformity.