Plating electroless Ni-P (EN) coating on titanium alloys is a complex process involving intricate pretreatment steps using strong acids that are dangerous to the operators, demanding on the operating conditions and harmful to the environments. To address this issue, this paper proposes a novel in-situ activation method where the substrate is activated in the plating bath. The microstructure and the chemical composition of the EN coatings were studied, the results showed that the coating exhibit typical semi-amorphous structure with ~13 at.% phosphorus content. The relationship between the dosage of the activator and the structure of the coating was investigated and the optimum dosage of activator was determined to be 20 mL/L with respect to the bath. Based on the results, the mechanism of this activation method was also discussed.