By analyzing morphology, weight gain, pitting depth, potential and AC impendence in corrosion course, initial pitting behavior of LY12 aluminum alloy in simulated marine atmosphere was studied. Results indicated initial pitting of LY12 was affected much by Cl- sedimentation and atmosphere humidity, little by temperature below 35 degrees C. Weight gain and pitting depth increased with rising of Cl- sedimentation quantity, humidity and temperature. Correlation of weight gain and time fitted Model Boltzman. In initial pitting course of LY12, open circuit potential changed little and Nyquist graph had the characteristic of adsorbent resist system.