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Galaxy mergers are usually identified by asymmetry morphologies, such as tidal tail, or peculiar dynamics with predominant random motion instead of rotation. In fact, in addition to the influence on the morphology and dynamic properties, galaxy-galaxy merger events also affect chemical compositions of stars dramatically. In particular, stars formed during a gas-rich merger tend to have lower iron abundance due to metal-poor gas accretion and enhanced alpha-to-iron abundance ratio because of the star burst triggered by the merging event. Recently, we identified two stellar populations in Milky Way with relatively young stellar age showing this peculiar chemical composition based on the SDSS/APOGEE survey. The two merger events occurred at ~8 (z~1) and ~4 (z~0.4) Gyr ago, respectively. Using a numerical chemical evolution model, we simulated the distribution of stars in [a/Fe]-[Fe/H] diagram predicted by the merger scenario. We found that the well-known alpha abundance dichotomy of stars in Milky Way could be well explained by the model involving gas-rich mergers. The first merger event occurred at z~1 triggered a star burst that forms the majority of the chemical thick disk stars observed today with [a/Fe]~0.3. In other words, our model suggests a merger origin of the thick disk in the Milky Way. The second merger at z~0.4 leads to the formation of young, alpha-enhanced ([a/Fe]~0.1) and iron-poor ([Fe/H]-0.3) stars in the thin disk which contributes to ~10% of the Milky Way disk in mass.