The invention relates to a giant magnetoresistance effect based sensor for measuring directions and intensity of magnetic fields, belonging to the technical field of magnetic field measurement. The sensor comprises an X sensing unit for measuring the magnetic field component in the X direction and a Y sensing unit for measuring the magnetic field component in the Y direction, wherein each sensing unit adopts two-stage bridge structure; the first stage bridges adopt the same constant-voltage power source for supplying power; the second stage bridges adopt the output voltage of the first stage bridges for supplying power; the first stage output bridges output the voltage generated by the measured magnetic fields; and the second stage bridges output the voltage which is the square number with the voltage generated by the measured magnetic fields. The sensor is suitable for measuring the intensity and directions of the unknown magnetic fields, can be applied to measuring such magnetic fields as geomagnetic fields, magnetic fields in the transmission lines and the substations and the like, can be used for measuring the current in the power systems and the electronic systems, has small volume, high response speed and high sensitivity and greatly improves the measuring frequency range and response speed of the sensor.