In view of the difficulty in stably meeting the ultra-low flue gas emission requirements of the wet flue gas desulfurization system of a 5000t/d cement clinker production line, this paper carries out a technical renovation study on the capacity expansion and upgrading of the wet flue gas desulfurization (WFGD) system. By comparing three renovation schemes, namely single-tower double circulation, double-tower series connection, and in-situ single-tower capacity expansion, the in-situ single-tower capacity expansion scheme is selected. The main renovation measures include elevating the desulfurization tower body, adding new spray layers and circulating pumps, optimizing the oxidation air pipe network, replacing the high-efficiency three-stage ridge-type demister, and raising the operating liquid level in the tower.After the renovation, the system runs stably. Under the working condition with the inlet SO₂ concentration no more than 2500mg/Nm³, the outlet SO₂ concentration is steadily lower than 35mg/Nm³, the dust concentration is less than 10mg/Nm³ and the desulfurization efficiency reaches 98.97%. All indexes fully comply with the ultra-low emission standards for the cement industry. Featuring a high equipment reuse rate, small floor area and convenient construction, the proposed renovation scheme can provide an engineering reference for the desulfurization system upgrading of cement clinker production lines with site space constraints.