The invention discloses a DC overhead transmission line sag and stress monitoring method based on weak magnetic detection. Multiple detection points are selected on a tower rod, magnetic sensors are placed, and by use of the distribution feature of a transmission line space magnetic field, in a corresponding coordinate system, a fitting linear equation formula between a magnetic field component generated by each transmission line at each detection point and a sag of each transmission line is obtained; and based on this, through coordinate conversion and a magnetic field superposition principle, a system of non-homogeneous linear equations is constructed by use of a differential method, sags of the transmission lines at the two sides of a pole and horizontal stress of the transmission lines can be solved, and finally, the size of stress at any point of the transmission lines is solved so that the sags and the stress of the transmission lines are monitored in real time. According to the invention, through a weak magnetic detection technology, through combination with a wireless communication technology, magnetic signals acquired by the magnetic sensors are uploaded to a monitoring terminal and then the sags and the stress of the lines are calculated in real time through a computer, such that faults can be simply and quickly discovered and eliminated.