Abstract: Based on the state addition principle，this thesis makes an initial analysis of the relationship between the intensity of light filtering through rubidium bubble and the constant magnetic field at the centre of the bubble，and expounds on the magnetic field determination method by optical pumping magnetic resonance experiment. The thesis also gives out the value of geomagnetic field in Xiangxi district.

Key words: optical pumping；magnetic resonance；light intensity；constant , magnetic field；geomagnetic field