A moderate-temperature desulfurization process at 600-800 degrees C was studied in a pilot-scale circulating fluidized bed flue gas desulfurization (CFB-FGD) experimental facility with the addition of the internal structure. The rapidly hydrated sorbent and the desulfurization products were analyzed to clarify the influence mechanism of the internal structure on the moderate-temperature desulfurization process. The results show that the desulfurization efficiency with the internal structure at 600-800 degrees C was 74-93% for the calcium to sulfur (Ca/S) molar ratio of 2.0, which was higher than the desulfurization efficiency without the internal structure of 67-83%. As for the calcium-containing compositions, the desulfurization products included about 70% cyclone recirculation sample and about 30% bag filter sample. The cyclone recirculation sample was mainly composed of CaSO4 and CaO. The bag filter sample not only contained high contents of CaSO4 and CaO but also contained a considerable amount of CaCO3 and even some unreacted Ca(OH)(2). With the addition of the internal structure, the calcium conversion rate of the cyclone recirculation sample greatly increased from 42.8 to 47.4% while that of the bag filter sample just increased from 29.4 to 30.7%. It demonstrated that the main contributor for the improved desulfurization efficiency was the cyclone recirculation sample clue to the improved solids concentration distribution and the enhanced gas-solid contact efficiency. The calcium conversion rate for the bag filter sample depended on the solids concentration distribution as well as the particle residence time in the moderate temperature range, which indicated that prolonging the particle residence time for the fresh sorbent and the fall off calcium-containing particles was important to further improve the desulfurization performance. These results provided good guidance for realizing high desulfurization efficiency and low flow resistance in dry FGD processes.