Abstract: A magnetic field measurement instrument based on GMI effect is developed by using Co_66.3Fe_{3. 7}Si₁₂B₁₈ amorphous ribbon annealed by pulsed current. The signal processing circuit of the sensor and the hardware circuit of the measurement instrument are designed,and the experiment result is given.The result shows that this instrument can measure the weak magnetic field ranging from-260 to + 260 A/m. It can be used in the weak magnetic measurement field including earth magnetic field and environmental magnetic field.