The problem of end face cracks in high-quality carbon steel continuous casting round billets with a diameter of 1 200 mm was investigated， with a production process.of 80 t BOF-LF-VD - ultra large size continuous casting machine - slow cooling of castings Measures such as multi-stage electromagnetic stirring， online insulation of castings， and slow cooling process were adopted. The continuous casting process parameters are as follows： casting speed of 0.06-0.11 m/min， tundish superheat of 15-35 ℃， continuous casting round billet charging temperature ≥ 500 ℃， and discharging temperature ≤ 350 ℃. Perform multidimensional correlation analysis using methods such as low magnification testing， scanning electron microscopy， and in situ analysis.Using TWS software and liquid core control model， the pulling speed parameters were optimized from 0.10 m/min to 0.07 m/min while keeping the electromagnetic stirring and superheat parameters unchanged.thereby improving the core quality of the casting billet and reducing low-magnification inspection center cracks from 1.0–2.0 levels to 0–0.5 levels. By using an online insulation cover， the end temperature of the continuous casting round billet increased by 102.4 ℃， and the slow-cooling pit capacity rose from 50% to over 80%. The cooling rate in the first two days decreased by more than 1 ℃/h. Inspection results showed that the crack length on the end face of the continuous casting round billet was reduced from 60–200 mm to 0–10 mm. Customers reported satisfactory results， and the improvements were effectively verified by the market， providing valuable reference for the industry.