Calculation of tidal changes reveals that the MS 7. 0 Lushan County，Sichuan，China， earthquake of April 20，2013 occurred at the minimum phase point of tidal force． It indicates that the seismogenic fault on which the tidal force acts on is of thrust type． The outgoing long-wave radiation (OLR) is the energy radiating from the Earth as infrared radiation at low energy to space． According to the tidal cycle，abnormal OLR change is analyzed based on NOAA satellite data around the whole of China before and after the earthquake． The result shows that the OLR changed evidently with the tide force change． Temporally，the change went through the course: initial OLR rise→ strengthening→ reaching abnormal peak→ attenuation → returning to normal; in space，the abnormal area was distributed along the Longmenshan fault and evolved as: scattering→ convergent → scattering． The process is similar to the change process of rock breaking under stress loading． It indicates that the celestial tidal force can trigger earthquakes when the tectonic stress reaches the critical break point of an active fault and the OLR anomaly is proportional to the seismic tectonic stress change． It is of practical value to combine OLR and tidal force anomaly with earthquake precursor studies．