Abstract: In this paper,the basic theory of precision estimation about lateral piercing errors in the GPS network of railway tunnel has been discussed. According to the theory, the software for computation has been made and tested with data.Combined with the piercing errors for different lengths of tunnel GPS network has been analysed.In the end,a few basic rules about the consfraction of the GPS network in the railway tunnel and some methods for rough estimation in lateral piercing errors is GPS network of railway tunnel have been proposed.