The online height measurement for a gas storage cabinet in the biogas project is great significant to its security control, estimations of biogas production and management of biogas consumption information. An online measurement of height called reed pipe online height measurement system which satisfied the demand of online measurement of the biogas holder was designed. The system was consisted of a permanent magnet, benchmarking measurements of reed switch and data acquisition boards. The magnet was mounted on the side of the counterweight frame. Benchmarking was a composition of multilevel with reed sensor group， and achieved data transmission through the chip concatenation. Data acquisition board was SCM system and the main role was to capture and upload the high signal. The system increased the enhanced cascade circuit to solve signal attenuation which was cased by the multilevel, in order to protect the system to utilize in the largescale degree measurement of a gasholder high test. Take constantpressureflexible biogas holder as an object, the system debugged, and results showed that the system had good stability and met the measurement accuracy practical engineering requirements.