Using density functional theory (DFT) calculations, we have comprehensively investigated the structure and the hydrogen evolution reaction (HER) catalytic activity for pristine and monovacancy defected WP systems. It was revealed that the (101) surface can have the most exposure for the WP structure. The calculated free energy values of H* (ΔG_H*) show that the (101) surface can exhibit good HER catalytic activity, where the top site over the W atoms can act as the most active site for HER due the existence of antibonding characteristics after adsorbing H*. Moreover, we have proposed an effective strategy through the introduction of a monovacancy to further improve the HER activity of the WP system. It was found that the formation of the W-monovacancy can significantly improve HER activity since the decreased coordination number of the correlative atoms brings some new active sites around the defect. Particularly, these systems can even exhibit considerably high HER activity over a wide range of hydrogen coverage. Clearly, all these fascinating findings at the atomic level can be beneficial for realizing highly efficient nonprecious HER electrocatalysts based on tungsten phosphide and even other transition metal phosphides in the near future.