This study aims to estimate the risk factors of gastrointestinal (GI) bleeding in patients with acute coronary syndrome (ACS) and to evaluate the optimal duration of dual antiplatelet therapy (DAPT). We enrolled 1266 patients with ACS in a telephone follow-up program to determine whether any of the patients were hospitalized for GI bleeding. We collected baseline data, laboratory tests, electrocardiograms, and echocardiography covering all ACS patients. Multivariable regression was performed to adjust for confounders and predictors of GI bleeding. At the same time, the optimal duration of DAPT for ACS patients was evaluated. A total of 1061 ACS patients were included in the study. After 13-68 months, 48 patients (4.5%) were hospitalized for GI bleeding. The risk of GI bleeding was significantly increased in patients treated with DAPT for more than 18 months (hazard ratio 12.792, 5.607-29.185, < 0.01). Receiver Operating Characteristic curve showed that the duration of DAPT using a cutoff of 14.5 months resulted in a sensitivity of 66.7% and a specificity of 77%. In patients with ACS, DAPT time are the main risk factors of GI bleeding. The optimal duration of DAPT is 14.5 months.