At present, people recognized that the gestation and occurrence of earthquake is caused by the movement of the medium created by the accumulation of underground stress, which also brings the change of medium's electrical properties, as a result it will make geomagnetic field change. Some relevant scholars had used geomagnetic method to study Wenchuan earthquake, but before the earthquake, they did not found good correlation between the earthquake and the change of earth magnetic field at epicentre. According to the relevance between earth magnetic field and the earthquake, this paper has discussed the correlation between the change of earth magnetic field at epicenter and Yushu earthquake, and the result showed that the correlation is corresponding. In the paper, we main used observations of geomagnetic within the scope of 500 km around Yushu before the earthquake, moreover, we also used a series of methods to studying this question, such as the geomagnetic vertical component of load-unload response ratio, the geomagnetic vertical component ratio of daily variation, the geomagnetic daily variations of the vertical component of spatial correlation, and the low point displacement. According to the piezomag-netic effect theory, stress change will cause the change of underground rock magnetism, resulting in the magnetic field change. Therefore, before an earthquake occurs, the slowly accumulation process of underground stress may cause the change of rock magnetism in seismogenic region and its nearby underground, and it also brings mutation of geomagnetic field during the earthquake. According to the expansion of magnetic theory, accumulation process of seismic strain can cause geomagnetic anomaly state of medium in large range and various physical changes arising from the process. When strain accumulates to a certain degree before the earthquake, rock will produce microcrack, increase porosity, which can lead to the expansion of the volume. If there is fluid in the rock, the expansion may lead to the spread of the fluid. Therefore, the process will cause the relative change of electric potential in the diffusion zone, thus it makes geomagnetic field change. Daily variation of geomagnetic field is a short-term change, it mainly controlled by the current system of ionospheric about 100 km altitude, and the current system is controlled by the sun. The generally accepted view is that, when there is a seismogenic zone, physical properties of the underground medium such as conductivity will change because of the stress function, especially appearing buckling stage before the impending earthquake. This change in conductivity can cause the change of variation magnetic field, leading to abnormal change in the form of daily variation of geomagnetic field. And diurnal variation of geomagnetic vertical component will deviate from normal background field, it shows the abnormal change of amplitude and phase. The anomalous characteristics in daily variations of the vertical component in spatial distribution is called geomagnetic diurnal variation anomaly field. Practice shows that predicting earthquake by geomagnetic diurnal variation anomaly field (such as geomagnetic low-value displacement method) has certain prediction effect, and there is a good correlation between this kind of rare anomaly and earthquake.