Aiming at damage detection for thin metallic plates, the identification of micro cracks is done by analyzing the characteristics of reflection, refraction and mode conversion of nonlinear Lamb waves at the defects, and especially nonlinear characteristic parameter β (the ratio of the amplitude of second harmonic A2 to the amplitude of fundamental-frequency wave A1). A finite element model of an isotropic aluminum plate with double crack is presented to study the role of nonlinear lamb waves in damage detection. The model considers the nonlinear characteristics of crack contact position and aims to explore the influence of the distance between the double crack and their orientation on the nonlinear characteristic parameter of the Lamb waves. The results show that, within a certain range, the increase of the distance between double crack leads to the increase of nonlinear characteristic parameters of Lamb waves. When the orientation of double crack are both parallel to the propagation direction of excited Lamb waves, the value of nonlinear characteristic parameters of Lamb waves is the largest number. In this work, the feasibility of using nonlinear Lamb waves for detection of double crack in thin plates is verified by numerical simulation, and the effects of the distance and orientation of double crack on the propagation of Lamb waves in plates is explored as well, which provides a good foundation for subsequent nonlinear damage testing of thin plates with randomly distributed micro cracks.