Abstract: Based on the observations of the baseline vector measured by GPS technology, a formula of accuracy estimation of lateral piercing for GPS tunnel control networks was deduced. A practical example verifies that the estimated results obtained by ignoring the covariance of the observations are very close to those by strict calculation taking the covariance into account. When estimated by inherent standard deviations of GPS receivers, the accuracy of lateral piercing can also meet the needs of a horizontal tunnel control network. This method of accuracy estimation of lateral piercing can adapt to the design of a GPS control network before tunnel construction surveying. The research result shows that the key factor to affect the lateral piercing accuracy for a tunnel is the relative accuracy of baseline azimuth surveying at the two entrances of the tunnel.