In order to further improve the precision and quantify the environmental requirement for Point Source Transmittance(PST) test, a method was proposed to quantitatively analyze the impact of air cleanliness on the precision of PST test. This method is based on Mie scatter theory, and uses Monte Carlo simulation to perform a random and large simulation on dust quantity and size for different air cleanness. A case study was performed to find out the actual environmental requirement for PST test, which used a space telescope of 800mm diameter with the designed PST of 10－9. The analysis indicates a minimum air cleanness of ISO class 6 and the PST test error of 10－10 caused by dust, with each additional level of air cleanness, the PST test error will increase ten times.