The development of the Horizon Directional Drilling and the technical specification of some pipeline instruments are described in this paper. The distribution features of abnormal points of H_x,H_z about the primary electromagnetic field in different depths on the linear electric current are analyzed. The theory to ascertain the underground iron pipeline is proposed. Some probing technological measures in frequency electricity, current and long grounded pole to improve the accuracy of underground pipelines buried deeper than three meters are suggested here. The applicable condition for the determination of the depth of the rather deep underground pipelines using horizontal magnetic field intensity is proposed. Three examples of pipeline probing in Guangzhou are given in this paper, in which the pipelines cross very wide rivers and long distances and the depths are deeper than 10 m.