Despite huge efforts have been devoted to investigating ultrathin layers of two-dimensional (2D) transition-metal dichalcogenides (TMDs), their realistic applications in electronics and optoelectronics are hindered by limited scalability and uniformity of 2D thin layers. In this work, a two-step synthesis method was adopted to produce wafer-scale molybdenum diselenide (MoSe2) nanosheets. Molybdenum oxide (MoO3) thin film was initially prepared via atomic layer deposition (ALD) and followed by a selenization process in chemical vapor deposition (CVD) tube furnace. MoSe2 nanosheets with desired thickness can be obtained by tuning ALD cycles in preparing MoO3 layers. The synthesized MoSe2 films exhibited excellent layer controllability, homogeneity and wafer-scale uniformity. Few-layer structure of our MoSe2 with a polycrystalline crystal structure was verified by means of Raman and transmission electron microscopy (TEM) measurements. Moreover, arrays of MoSe2-based photodetectors with different device dimension were fabricated and the photo-responses of the devices were studied. The device exhibited a fast photo-response time of 50 ms, a high on/off ratio of similar to 24 and a good photo-responsivity of 11.7 mA/W, and it is found that the effective illumination area was a critical factor for application. The work opens up an attractive approach to realize the application of wafer-scale 2D materials in integrated optoelectronic-systems.