In order to study the propagation law of blast shockwaves in a square tunnel, TNT charges with different mass of 1.00 kg, 3.25 kg and 10.28 kg were detonated separately in the tunnel. The shock wave pressure-time curves at different distances were recorded and the parameters of shock waves such as the arrival time, the peak overpressure and the impulse were obtained. Results show that the propagation of shockwaves in tunnel is divided into three stages: the free-field spherical wave propagation, the one-dimensional planar wave propagation and the transition between them. The peak overpressures fit the Hopkinson-Cranz scaling law well except near the explosion center. An empirical formula for calculating peak overpressures was fitted, and the deviation between fitting results and measured values was not more than 10%. However, the impulse does not agree with Hopkinson-Cranz scaling law.