There are thermal infrared anomalies(TIA) before earthquake,and TIA has become one of the important parameters for assessing regional earthquake risk. However,not all of the surface infrared anomalies are related to tectonic activities or earthquakes. How to eliminate the influence of nonstructural factors and extract the weak signals from strong disturbances is the key and difficult point for tectonic activities studies based on the thermal infrared remote sensing techniques. Land surface temperature(LST) background field is the basis for thermal infrared anomalies extraction. However,the established background fields in previous researches cannot eliminate the influence of climate changes, so the accuracy of thermal anomaly extraction is limited. Now an improved method is proposed. Combined with the periodic character of LST time series,harmonic analysis is lead into the process of LST background field establishment. Specifically,the yearly trend of LST is fitted based on Fourier Approximation method. As a new background field,the yearly trend is dynamic,includes the local and the yearly information. Then,based on the rule ofksigma,the earthquake anomalies,calculated by RST with the yearly trend of LST,can be extracted. At last,the effectiveness of the algorithm can be tested by the quantitative analysis of anomalies with anomaly area statistics,anomaly intensity statistics and distance index statistics. The Wenchuan earthquake was discussed again based on the proposed algorithm with MODIS land temperature products in 2008. The results show that,there were obvious pre-earthquake thermal anomalies along the Longmen Mountains faults with a longer time; but there were no anomalies when the earthquake happened; and the post-earthquake thermal anomalies occurred with much smaller amplitudes and scopes. Compared with the results derived from the traditional RST which is based on the spatial average of LST values,the TIA extracted by the new RST,which is based on the yearly trend of LST,is more fit with the active faults,and the process of the anomalies occurring and removing can be described in more detail. Therefore,as the background field to extract earthquake anomalies,the yearly trend of LST is more reliable.