On July 22, 2013, an earthquake occurred in Minxian County, Gansu Province, whose epicenter was located 154 km from the Tianshui seismic station. Geomagnetic three-component second sample observations at Tianshui station were initiated in 2007 through a GM4 fluxgate magnetometer. Here, we analyze the change of the geomagnetic polarization value and the transform function at Tianshui station from 2008 to 2014 based on the second data of GM4 fluxgate magnetometer. The results indicate that the geomagnetic polarization value and the transform function show different degrees of anomaly before the Minxian MS6.6 earthquake.(1) The transform function exhibited down turning recovery before the earthquake. In fact, the earthquake occurred just as the transform function reached the maximum.(2)The short-term increase was more obvious than the long-term trend, and the rising amplitude was larger.(3) Before the earthquake,the Parkinson vector azimuth in the periods of 20 s, 50 s, and 80 s was rotated and directed to the epicenter.(4) In April 2013, three months before the earthquake, the geomagnetic polarization at Tianshui station showed two anomalies of high values exceeding three times the mean square deviation.