Abstract: α spectrum method is one of the most important methods in measurement of ²²²Rn/²²⁰Rn concentration in environment. However, the peak tailing from high energy particles is an obstacle for accurate measurement. To improve the accuracy of measurement, study on the mechanism and effect of the tailing were carried out, and calibrating experiments on peak tailing correction factors were also realized using ERS-2 monitor. The peak tailing correction factors and calibration factors of ²²²Rn/²²⁰Rn were measured by experiment and their accuracy were also tested. It is suggested that during calibrating α spectrum monitor of ²²²Rn/²²⁰Rn activity concentration, the peak tailing correction and calibration factors should be recalibrated if necessary.