Directed Energy Deposition (DED), known as one of the additive manufacturing technologies, can be utilized to manufacture 3D metal parts via layer-wise cladding, and thus it provides an opportunity to manufacture complex-shaped and customized parts. Besides, Ti-6Al-4V has become one of the most widely studied alloys due to its high strength to density ratio, good corrosion resistance, biocompatibility, and weldability. We set up a coaxial camera with the nozzle (LAMDA 200, Mitsubishi Heavy Industries Machine Tool, Ltd.) for in-situ monitoring of the molten pool surface. Characteristics of the molten pool like Nozzle Brightness Ripple are applied to recognize the surface unevenness. Meanwhile, computational thermal-fluid dynamics (CtFD) simulations are also performed to analyze the melt dynamics, which agrees with results of the in-situ monitoring. Finally, a Proportional-Integral-Differential (PID) feedback system is utilized to accommodate processing parameters and thus improve the surface properties of Ti-6Al-4V fabrication.