Analysing characteristics of geomagnetic anomalies before M_S≥5 earthquakes in recent years in Gonghe basin and at middle east section of Qilian mountains and space time distribution of the anomalies, geomagnetic short term prediction indexes before strong earthquakes in northwestern China are studied. The results indicate that various degrees of anomalies of magnetic storm, geomagnetic low value displacement and correlation coefficients of geomagnetic vertical component between stations appeared before strong earthquakes. Interval from beginning of geomagnetic low value displacement anomalies to origin time is 10～40 days. Interval from magnetic storm to origin time is three days before and after the dates which are multiple of nine. Coming earthquakes are located at near line of demarcation of the low value displacement anomalies, or in the anomalous area. If duration of the correlation coefficient anomalies is over 5 months, it is possible that M_S≥5 earthquake occurs in anomalous area.