In this study, the wall thickness of the desulfurization tower during operation was measured using the conjugate gradient method according to the infrared thermogram of the tower surface. Because of the harsh detection environment inside the desulfurization tower, the inner-wall heat flow, which was needed for wall thickness detection, was also obtained by inversion of the conjugate gradient method. First, the feasibility of the proposed method was verified using numerical experiments. Subsequently, based on the infrared thermal image inversion, it was found that the desulfurization tower had some problems, such as the corrosion of the cylinder, the falling off of the anticorrosive coating, and the thinning. In the subsequent downtime maintenance, the problem area was rechecked, and the anomalies mentioned above were confirmed, which indicated the effectiveness and accuracy of the quantitative detection method for the wall thickness of the desulfurization tower based on the surface infrared thermal image proposed in this study.