Some ultrafine Ni-B amorphous alloys with variable B contents were prepared by chemical reduction under different conditions. The effect of the B content on their activities was evaluated by using the liquid phase cyclohexene hydrogenation as a probe. Both the specific activity and the TOF value increased with the increase of the B content, showing the promoting effect of the alloying B on the hydrogenation activity, which was mainly attributed to its modification of the nature of the Ni active sites. Concerning the structural effect, the XRD patterns revealed that the amorphous degree of the as-prepared Ni-B alloy increased with the increase of the B content. Meanwhile, the DSC analysis demonstrated that increase of the B content could increase the thermal stability of the Ni-B amorphous alloy and in turn, delay its crystallization during the cyclohexene hydrogenation. Furthermore, the EXAFS also demonstrated that the Ni active sites became more highly unsaturated at higher B content. These structural modifications were favorable for the hydrogenation activity. Concerning the electronic effect, the XPS spectra demonstrated the electronic interaction between Ni and B in the Ni-B amorphous alloy, making Ni electron-enriched while B became electron-deficient. According to the DFT calculations, more electrons may transfer from the alloying B to Ni with the increase of the B content. Such electronic modifications may also facilitate the cyclohexene hydrogenation.