Abstract: The concept of repetitively-pumped times of atomic iodine in COIL was introduced. According to the law of energy-conservation, an equivalent lasing model was built. The non-extractable residual energy while lasing, the relaxation period for upper energy-level of atomic iodine and the energy-transfer period of collision between singlet oxygen and atomic iodine were calculated. The calculation formula for COIL output power agrees better with the experimental results than the original one does. Both theoretical and experimental results indicate that: if the iodine flow rate is too low, the laser gain is less than the loss, and the laser cannot start operating. If the iodine flow rate is increased gradually, the laser output power increases accordingly, and the repetitively-pumped times of the