Abstract: To prevent the risk of displacement induced by shield tunneling, it is necessary to conduct probabilistic analysis. Gap parameter explains the source of ground loss in detail by dividing the ground loss into three parts. Based on it, the uncertainty of many parameters in the shield tunneling process is characterized, which provides an important prerequisite for probability analysis. The main idea is to obtain the statistics of three components of the gap parameter: three-dimensional equivalent gap u^*_3D, gap due to workmanship ω and physical gap g. The three-dimensional equivalent gap u^*_3D and the gap due to workmanship ω are relatively complicated, and the statistics of the relevant geological and tunneling parameters are given according to the literatures. The physical gap g is directly related to the grouting effects. The filling rate of grouting is determined by the geological radar data, and the statistics of the physical gap are given.