Negative air ion concentration is an important criterion for judging air quality. The traditional negative air ion concentration detector measures the current formed at the two ends of the collector plate by a capacitive ion collector, and calculates the concentration by the relationship between the measured current and the negative ion concentration. The negative air ion concentration detector designed by this method can only measure the concentration of a limited number of radius, and there is a certain systematic error in measuring the concentration of a large radius. In order to meet the requirement of high precision detection of negative air ion concentration, a method of detecting negative air ion concentration based on mobility analysis is proposed by simulating and analyzing the trajectories of air ions with different radius in the collector. The corresponding circuit system and design parameters are given, and the rationality of the system design is analyzed and demonstrated. This method combines current detection with numerical calculation, and can accurately measure the concentration of negative ions with different radius in the air. It has important application value in the field of environmental monitoring and assessment.