<p>Bilateral internal thoracic artery (BITA) grafting has been associated with improved survival and reduced graft occlusion, yet competitive flow (CF) remains a major cause of arterial graft failure. We used numerical simulations to investigate CF and the probability of graft failure in the in-situ BITA method. Patient-specific coronary geometry was reconstructed from CT scans of a 56-year-old male, and different stenosis severities were simulated. The results showed that graft performance was strongly dependent on the degree of stenosis. At 95% LAD stenosis, the mean LITA graft flow (MGF) was 0.595 cc/s, with a pulsatility index (PI) of 0.92, diastolic filling (DF) of 41%, and systolic reverse flow (SRF) of 0.7%, indicating favorable graft function (MGF &gt; 0.34 cc/s, PI &lt; 5, DF &gt; 25%, SRF &lt; 3%). At 80% stenosis, MGF decreased to 0.177 cc/s, PI rose to 6, DF remained ~ 42%, and SRF increased sharply to 27%, consistent with marked competitive flow. At 60% stenosis, graft performance deteriorated further (MGF = 0.081 cc/s, PI = 10, DF = 45%, SRF = 40%), suggesting conditions strongly predisposing to graft occlusion. Similar reductions were observed in the RITA graft (0.28 cc/s at 95% stenosis vs. 0.15 cc/s at 80%). FFR decreased with stenosis severity (0.82 at 70%, 0.78 at 80%, 0.65 at 95%), validating the hemodynamic significance of severe lesions. Hemodynamic indices confirmed the risk of graft failure in stenosis ≤ 80%; for example, the oscillatory shear index exceeded the critical threshold at multiple sites under 80% stenosis, indicating disturbed flow and potential long-term graft failure.</p>