region-time-length algorithm(RTL) is computing method that detect relative change of earthquake activity level. Using this method, four earthquakes(M_D ≥ 5.0)that occurred in Fujianand its offshore area for the past few years have been tested retrospective in this paper. the result show that, research starting time of every earthquake is determined by its V_RTL background activity level respectively, mean square error of the four earthquakes have all passed 3.5σ before shock , V_RTL  maximum value of three earthquakes have passed 150,one has 21.6 only. This illustrates that V_RTL  activity level have all marked variation relative to background activity level before M_D  5.0 earthquakes in this area, V_RTL  may be a index of earthquake prediction in this area.