Based on the crack extension testing of 7075 Aluminum - Alloy CCT specimens in three kinds of thickness under random spectra, the fractures of the specimens are carefully analyzed by virtue of election scanning microscope in this paper. Through the comparison of the photomicrographs with the same magnification factor and at the same length of corresponding crack positions of specimens different in thickness, it is found that the three-dimensional geometry of specimen has a great effect on the crack growth rate; and the thicker the specimen , the wider the fatigue strap, and the faster the crack extends. By the end of this paper, the mechanism is analyzed.