The second-generation single-crystal superalloy DD6 has a series of merits, such as high-temperature strength, combination properties, structural stability, and better casting performance. It is a good choice for manufacturing turbine blades. A reliable joining technology for the single-crystal superalloy DD6 is important for engineering applications. In this study, a newly designed Ni-based filler alloy with low boron content was used to join the DD6 superalloy. To avoid the formation of brittle borides within the joint, boron was reduced. However, the element Pd was added into the filler alloy as a melting-point depressant. The brazing process can be conducted at 1220^oC, which was lower than the solution treatment temperature of the DD6 base material. The effects of the gap size on the joint microstructure and mechanical properties were investigated. After brazing with the new Ni-based filler alloy, the matrix of brazing seam was γ + γ′ dual-phase, which was similar to the DD6 base material. The brittle borides in the joint were increased because of the big gap size, and the morphology of the borides was transformed from the discontinuous strip to coarse fishbone. When prefilling the FGH95 superalloy powder in the brazing seam with gap size of 0.15 mm, borides were refined and dispersed. With the increase in the gap size, the joint strength increased and then decreased. The γ + γ′ dual structure was refined when the gap size increased from 0.05 mm to 0.10 mm. Moreover, the content of elements Al, Ti, and Ta was high in the matrix of the joint with a gap size of 0.10 mm, which can strengthen the γ′ phase. When the gap size was 0.15 mm, the joint strength decreased because of the coarse borides. The highest joint strength was obtained when the gap size was 0.10 mm, and the average joint tensile strength tested at 980^oC was 694 MPa. After ageing heat treatment, the morphology of γ + γ′ was modified and the joint tensile strength increased to 807 MPa.