Abstract: Measuring radon exhalation rate from materials surface by activated charcoal canister is a common method at home and abroad. In this method, the detection efficiency of activated charcoal canister is essential but difficult to be determined because of the lack of ready-made instruments. A new method was presented to calibrate the detection efficiency by avoiding determining the radon activity of activated charcoal canister. By means of radon activity released from activated charcoal canister and the net γ-ray counting rates of this canister before and after exposure, the equation of detection efficiency was derived and the corresponding calibration method was designed. Using this method, the detection efficiency of activated charcoal canister with diameter of 63 mm and height of 32 mm was calibrated for 5 times, and the average result is 6.57%±0.26%. Monte Carlo method was used to simulate the above-mentioned activated charcoal canister. The simulation result of detection efficiency is 6.92%, which is in agreement with the result of experiments by the proposed method with error of 5%. The above results show that the presented method is accurate and reliable.