The influence of DC transmission lines on the geomagnetic observation is the urgent problem. Based on the principle of geomagnetic observation instrument, the influence is analyzed, and it is believed that the magnetic field of current on DC transmission line which is attached to the observed magnetic field of geomagnetic observation instrument is the main reason. According to the direction of current on DC transmission line and the relative position between geomagnetic station and transmission line, the influence characteristic is researched. A calculation model for magnetic field of current on polar conductor and a model for ground current in infinite homogeneous soil and in layer soil around earth electrode field are established. Calculated results of the magnetic field strength at different distance from earth electrode and polar conductor show that main influence of DC transmission line on geomagnetic observation is its influence on Z component of geomagnetic, and the influential degree is related with imbalance current of DC transmission line. During the calculation of the influence, the soil conditions and the ground current could be ignored.