In this paper a dyeing method using organic dye is presented to detect the dimension and shape of surface crack under fatique progression. The results show that it is an effective method for detecting the depth of surface crack in Aluminium alloy thin-plate. Several other problems in elastic-plastic fatique progression of surface crack. such as the yielding on back surface，the changing role of crack shape and the crack penetration points are studied. Finally the fatique data are treated by line-spring model and some conclusions are drawn.