The invention provides a method for measuring wall thickness of a pipeline, which comprises the following steps: a detector is disposed on the pipeline to be measured, and the detector is connected with a main controller; echo signals are acquired by using the wall surface of the probe tube of the detector, and the echo signals are transmitted to a main control computer; the main control computeris used to determine the corrosion area of the pipeline according to the echo signals; a phased array detection assembly is arranged on the corrosion area, the phased array detection assembly is connected with an upper computer, the corrosion area is scanned through the phased array detection assembly to acquire defect conditions, scanning results are transmitted to the upper computer, and the upper computer is used to the scanning results and form the images of the scanning results; and the upper computer is used to calculate the minimum wall thickness of the corrosion area according to the image forming condition. According to the method, by adopting a combination of guided wave rough scanning and phased array detection accurate measurement, wall thickness condition of fire-fighting water pipelines can be measured quickly and efficiently, and the measured data are accurate and reliable.