Using a simple method of direct heating of bulk copper plates in air, oriented CuO nanowire films were synthesized on a large scale. The length and density of nanowires could be controlled by growth temperature and growth time. Field emission (FE) measurements of CuO nanowire films show that they have a low turn-on field of 3.5–4.5 V µm−1 and a large current density of 0.45 mA cm−2 under an applied field of about 7 V µm−1. By comparing the FE properties of two types of samples with different average lengths and densities (30 µm, 108 cm−2 and 4 µm, 4 × 107 cm−2, respectively), we found that the large length–radius ratio of CuO nanowires effectively improved the local field, which was beneficial to field emission. Verified with finite element calculation, the work function of oriented CuO nanowire films was estimated to be 2.5–2.8 eV.