The simulated observation of water radon in China is cumbersome and has many interference factors. To overcome this drawback, a novel radon detector instrument was introduced in this paper, and a comparative experimental study was carried out with the FD-125 radon detector. The two water radon detectors were calibrated in the standard radon chamber of the emanometer measurement platform for earthquake monitoring, and a comparative measurement with the same water samples and environment was carried out at Jiujiang seismic station. The results show that the repeatability, relative error, relative inherent error, and inherent background of the novel radon detector can meet the technical requirements with a standing time of 1 h. The observation data has good consistency with those of FD-125 radon detector, but the sensitivity index needs to be improved. The overall design of the proposed instrument can basically meet the development needs of analog water radon observation and reduce the measurement error caused by manual operation, thus improving the measurement efficiency and data quality.