Using particle counting method to assess automotive filter filtration efficiency and retained capacity has been gradually accepted by filter industry, particle counting method is also beginning to gradually replace traditional weight method.But when the oil particles concentration increases to a certain degree, it will exceed the particle counter measurement repeatability when the particle counting method is used in testing.This will make the particle counter readings inaccurate, which affects the calculating value of filter filtration efficiency.Based on researching the test standards in which particle counting method was used to assess the efficiency of the filter at home and abroad, a set of online dilution system was developed .The on-line dilution system was used on the existing filter multi-pass test bench.The system could be used to accurate dilute oil according to the set ratio.Three levels dilution ratio, which were 1∶2, 1∶4 and 1∶9,were designed in the dilution system.It can satisfy the test of the existing filter filtration efficiency, and can pass the validation of the GB/T 21540-2008.