Abstract: Both radon and thoron exposure monitoring method at the same time, and thoron exposure only monitoring method with absorber discriminating radon are developed. A simple calibration system is developed too. When radon and thoron exposure are measured simultaneously, two same dosimeters with different filter materials have the different diffusion rate constants λ d, and are of different response factors for radon and thoron. The lower limits of detection are 0.74 and 1.25 kBq·m -3 ·h for radon and thoron exposure measurement, respectively. When both a piece of 45 μm thick aluminium film and 10 μm thick PE preservation film are placed on CR-39 SSNTD, all α particles emitting from radon and its daughters are absorbed, only a part of α particles emitting from 212 Po(ThC′) can be recorded by CR-39 SSNTD. The lower limit of detection for thoron exposure measurement is 22.1 kBq·m -3 ·h._