In this paper, we use five geomagnetic analysis methods including geomagnetic low-value displacement, geomagnetic daily value spatial correlation and spatial correlation of daily variation of geomagnetic vertical component, geomagnetic load-unload response ratio and daily ratio of geomagnetic vertical component amplitude to analyze the geomagnetic precursor anomalies before the Wenchuan M_S8.0 earthquake on May 12, 2008. The study found that:three methods of geomagnetic low-value displacement, geomagnetic load-unload response ratio of geomagnetic vertical component amplitude and daily ratio of geomagnetic vertical component amplitude had abnormal phenomena before the Wenchuan M_S8.0 earthquake. And the earthquake occurred in the joint region of the prediction lines. The analysis shows that comprehensive analysis by multiple methods can effectively improve the accuracy of earthquake prediction. The mechanism of the geomagnetic abnormal diurnal variation is also discussed.