In order to study the effects of surrounding rock void of the tunnel bottom on dynamic response characteristics of the heavy-duty railway tunnel under the hydro-mechanical coupling effect. The method of combining field test and numerical simulation was adopted. Taking the vertical displacement, pore water pressure, and vertical dynamic stress as evaluation indicators, the dynamic response law of water-rich tunnel bottom structure under different cavity shapes (ellipse, cosine and rectangle) was analyzed. A practical void shape has been proposed, and the threshold value of the tunnel bottom surrounding rock void in the water-rich tunnel was determined. The results show that with the increase of the void width, the dynamic response of the tunnel bottom structure increases continuously, and the dynamic response changes intensify when the void width exceeds a certain value, this phenomenon is especially obvious in the void area. Among the three void shapes, the rectangular void has the largest dynamic response, followed by the elliptical void, and the cosine void is the smallest. Among them, the elliptical void is the most realistic. The dynamic response of the base under this void is 4.2 times that of the case without void, and the threshold value of surrounding rock void of the tunnel bottom is 0.6 m. The research results can provide a basis for the safety of water-rich heavy-duty railway tunnels and the treatment of voiding diseases.