Abstract: Accurate positioning of underwater benchmark(transducer) is a key in a high precision positioning system,which influences ultimate quality of the acoustic positioning system.We make in-depth analysis of the positioning errors of the underwater benchmark due to cylinder deformation,positioning error of GPS,angular error of compass,and measurement error of the dynamic motion sensor.With reasonable assumption of the engineering data,we have obtained the result that the error of the underwater benchmark is a little more than 0.3 meter if the GPS error is less than 0.15 meter.The analysis provides useful data for reference of the error control in practical applications.