In recent years, organic-inorganic hybrid perovskite solar cells (PSCs) have attracted wide attention due to their high photoelectric conversion efficiency and simple preparation process. Hole transport layer (HTL) is one of the most critical components in PSCs. As a kind of inorganic HTL material, nickel oxide (NiO_x) has been widely used in perovskite solar cells because of its excellent advantages, such as outstanding chemical stability, high carrier mobility, simple methods for its preparation, etc. In this paper, the applications of NiO_x HTL in planar PSCs are systematically summarized from the aspects of the improvment of its structure and photoelectric properties by doping and interface modification. The reasons for affecting the device performances, i.e. fill factor, open-circuit voltage, short-circuit current, photoelectric conversion efficiency, and stability are emphatically analyzed from several aspects, such as energy level matching, hole mobility and crystallinity. In addition, the future development directions of the planar PSCs are prospected.