Abstract: The cross-hole resistivity tomography method has certain advantages in boulder detection in metro shield zones, but some practical engineering problems, such as low location accuracy and difficult identification of small-size boulders, still exist. First of all, the inequality constrained inversion imaging method and the parallel analytical fast solving method for partial derivative matrix are proposed for reducing the multiple solutions and improving the imaging results in cross-hole resistivity inversion. On this basis, a combined detection mode is raised from numerous results of inversion imaging comparison experiments by routine detection modes. The advantages of various routine detection modes and the collected data amount are integrated in this mode, which is good for improving the recognition ability of abnormal bodies. Then, numerical simulation tests for borehole distance and electrode spacing which affect the detection results are performed. Under the combined detection mode, the effective range of borehole distance and the reasonable electrode spacing parameters are obtained. Further tests on the identification effects of small-size boulders are carried out. On this basis, the boulder detection scheme, which combines the ground geophysical screening and cross-hole resistivity tomography detection, is put forward. Finally, physical model tests are carried out to verify this method. The detection results are almost the same with the actual situation, so the proposed method has good application prospects.