Based on experience obtained in the practical work of the demonstrative projects for China's critical mines, the authors have reached the following conclusions: For the new type JCX3 borehole threecomponent magnetic survey instrument produced by the Chongqing Geological Instrument Factory, when the inclined plane of the probe is directed northward, the positive value of the X numerical reading is the maximum; when the inclined plane is directed southward, the negative value of the X numerical reading is the maximum, and the X component is directed towards the borehole inclination. Although both of the above two situations accord with the left handed rule in the system of rectangular coordinates, the x, y coordinates of the borehole magnetic survey sensor of the new instrument are different from things of the old instrument by 90°. It is therefore thought that, for the measurement results obtained by tens of JCX3 type instruments which are extensively used at present, the utilization of the vector calculation formulae established on the basis of the vertical system might lead to the mistaken application and interpretation. And hence this problem deserves further consideration.