The utility model provides a protection structure for excavating underground water on a lower step of a soft rock tunnel. The protection structure comprises freezing holes excavated in the peripheralboundary of an upper step of tunnel excavation in a radial mode, a freezing device arranged on the upper step and a water pressing device. And each freezing hole penetrates through the permeable layerbelow the upper step and extends downwards into the water-containing layer at the bottom, so that frozen saline water is injected into the water-containing layer under the action of the water pressing device. According to the technical scheme, the inadaptability of a conventional underground water grouting and blocking method applied to soft rock engineering is solved, direct contact between softrock and underground water is eradicated, softening, deformation and disintegration of the soft rock when meeting water are avoided, and the construction speed and the engineering safety are effectively improved.