The influencing factors of light extinction automatic particle counters measurement results are studied,such as ultrasonic defoaming,negative pressure defoaming,positive pressure defoaming,the frequencies of pipeline cleaning and different test personnel.The results show that the method of ultrasonic,negative pressure and positive pressure can effectively defoam in the samples.The more clean the line of equipment,the less impact to the measurement results of contamination level.The above mentioned influencing factors have a marked impact to contamination level of the debris whose size is larger than 14 microns.