It is difficult to accurately predict the karst water burst by existing geological prediction technology during tunnel construction in karst region. Hence, a transient electromagnetic detection technology with large crosssection fixedpoint and threedimensional multismallline is designed. And then according to the karst development characteristics, the caves with different sizes 25 m and 50 m ahead of tunnel face are simulated by numerical simulation method, and the transient electromagnetic response characteristics of lowresistance anomalous bodies with different sizes and distances from tunnel face are studied. Finally, the engineering practice is carried out in Guzhang Tunnel on ZhangjiajieJishouHuaihua Railway to verify the simulation results. The results show that: (1) A closed lowresistance region with apparent resistance value less than 60 Ω·m is formed 25 m and 50 m ahead of tunnel face on 2D apparent resistivity contour map; and then the position and range of the lowresistance region is in accordance with those of design models. (2) The results revealed by transient electromagnetic detection technology with large crosssection fixedpoint and threedimensional multismallline are basically consistent with the actual situations; and the karst 17-30 m ahead of tunnel face and 20 m from both left and right side of tunnel to axial line can be accurately predicted by the abovementioned technology.