SAGD horizontal wells are used to enhance oil recovery from heavy oil reservoirs. This technology requires precise separation between the production well and the injection well to ensure the efficient drainage of the reservoir. By studying the attitude of the downhole probe tube and the production well trajectory, an algorithm is proposed for eliminating ferromagnetic interference while drilling injection wells. A high accuracy filter circuit has been designed to correct the detected magnetic signals, which are ultra-weak, frequency-instable, and narrow-band. The directional drilling magnetic guidance system (DD-MGS) has been developed by integrating these advanced techniques. It contains a sub-system for the ranging calculation software, a magnetic source, a downhole probe tube and a sub-system for collecting & processing the detected signals. The DD-MGS has succeeded in oilfield applications. It can guide the directional drilling trajectory not only in the horizontal section but also in the build section of horizontal injection wells. This new technology has broad potential applications.