Objective: The purpose of this study is to evaluate the changes of LA size and function by real-time 3D echocardiography (RT-3DE), and provide a better understanding about their effects on treatment and prognosis. Methods: Ten Bama miniature pigs were used for modeling STEMI. Images were obtained by RT-3DE at multiple time points. From the LA volume-time relation curve, the LA volume (LAV) and functions (storage, channel, and active emptying) at different phases were calculated. Blood samples were obtained for measurements of NT-proBNP, creatine kinase-MB(CK-MB) and cardiac Troponin-I(cTn-I). Results: (1) LAVmax, LAVmin and LAVpre-a were all increased with time. (2) The LA expansion index was the most sensitive index of LA functional change. It increased (p< .05 vs. baseline) at the 1st day after STEMI (acute phase), and was correlated with diastolic function E/E’ (p< .05), as shown by univariate correlation analysis. (3) On the 28th day after STEMI, the active emptying percentage of total emptying (AE) showed a continuously increasing trend (p< .05), and was correlated with VTILVOT (time-velocity integral of left ventricular outflow tract) (p< .05). Conclusions: During the acute phase after STEMI, LA volume changed in order to increase LV preload. After STIMI, the storage function of LA was the first to change, while active contractile function was significantly enhanced during subacute phase. With the accurate measurement of LA function with RT-3DE, our study will help improving the therapeutic target setting and pharmacologic interventions, which may enhance the clinical outcomes of STEMI patients.