This work derived the relationship of the emanation factor and the radon that released from building materials based on the equation of balance for each phase of radon in unbroken building materials

and calculated the emanation factor of the radon that released from the top-surface of the building materials by measuring the concentration curve of radon in the equipment at intervals.The radon attenuation in air-filled pores of building materials is taken into account.The measuring results are more accurate after the calculation of leakage rate of the equipment by measuring the concentration of the pre-injected radon in the equipment at intervals.A high accurate measuring formula for short time is also obtained for the existing of "back-diffusion" in long time measuring

which can accurately fit the experimental data within ±4%.