Tunnel engineering is a necessary part in the railway and highway constructions. The tunnel construction network established by using GPS has the advantages of high speed， high precision and low expenses. The calculation about the influence of tunnel GPS network to the lateral error of piercing is very important. F3asing on the conducted formula of tunnel GPS lateral mean square error of piercing，the authors present one set of data processing method about GPS used in tunnel engineering,and give a calculating example in the practical engineering .