With the method of differential optical absorption spectroscopy (DOAS), aerosol along the light path is measured with a flashlight source. After calibrating the system parameters by the visibility in the clear day, the difficulty to measure the original light intensity in aerosel measurement with DOAS is resolved, and the optical thickness from 350～650 nm is obtained with method of inversion. Comparison with solar photometer and good correlation is found between aerosol optical thickness and suspended particle matter concentration. In addition, the atmospheric visibility is retrieved by extinction coefficient at 550 nm.