Designing a stable and efficient dual-functional catalyst for the hydrogen evolution and oxygen evolution reactions (HER/OER) is of great significance to the development of hydrogen production by water splitting. This work reports on novel halogen (X = F, Cl, and Br)-doped carbon dots modifying amorphous cobalt phosphide (X-CDs/CoP), which can be tuned by the choice of X-CDs to have urchin, Pinus bungeana, and Albizia julibrissin type structures. The different characteristics of the various X-CDs led to different formation mechanisms and final structures. As a bifunctional catalyst, urchin-shaped F-CDs/CoP crystals achieve superior electrocatalytic performance, exhibiting excellent HER/OER activity and sustained stability in an alkaline solution. For overall water splitting, they provide current density of 10 mA cm−2 and require a low cell voltage of 1.48 V in 1 M KOH. In addition, the catalytic performance shows negligible degradation after 100 h, thus demonstrating excellent long-term cycling stability. Density functional theory calculations show that the improved electrocatalytic performance of F-CDs/CoP catalysts is due to the coupling interface between CoP and F-CDs, which optimizes the hydrogen/oxygen adsorption energy and accelerates the water splitting kinetics. This work provides guidance for the rational design of transition metal phosphide electrocatalysts with outstanding performance.