Abstract: Investigation of karst voids is significant to the construction in the karst area. Cross-hole seismic tomography is an effective borehole geophysical method. We conduct an inversion study on cross-hole seismic karst voids detection based on the Eikonal equation. Multiple numerical scenarios are used to test the effectiveness of the proposed method. FDTD is applied to simulate cross-hole seismic detection. The arrival time of the first break is extracted from modeling recorded wave profile. The Eikonal equation traveling time inversion is used to image the underground velocity between boreholes. Four scenarios including single void, single fracture, fracture and void, and multiple voids are analyzed. The result shows that the Eikonal equation-based inversion method can image the location, size and orientation of the karst voids and fracture. The inversion is stable even in complex scenarios. The velocity difference between voids infill can be identified. However, the inverted velocity is slightly higher than the true velocity. The lateral resolution is lower than that in depth. The inversion method is not sensitive to the fracture anomaly. The Eikonal equation-based inversion method is robust and efficient in complex scenarios with multiple anomalies, and can be applied in practice to provide important geological information.