Atomically thin MoS_2 films have attracted significant attention due to excellent electrical and optical properties.The development of device applications demands the production of large-area thin film which is still an obstacle.In this work we developed a facile method to directly grow large-area MoS_2 thin film on Si O_2 substrate via ambient pressure chemical vapor deposition method. The characterizations by spectroscopy and electron microscopy reveal that the as-grown MoS_2 film is mainly bilayer and trilayer with high quality. Back-gate field-effect transistor based on such MoS_2 thin film shows carrier mobility up to 3.4 cm~2V~(-1)s~(-1) and on/off ratio of 10~5. The large-area atomically thin MoS_2 prepared in this work has the potential for wide optoelectronic and photonic device applications. Atomically thin MoS_2 films have attracted significant attention due to excellent electrical and optical properties. The development of device applications demands the the production of large-area thin film which is still an obstacle. In this we we developed a facile method to directly grow large-area MoS_2 thin film on Si O_2 substrate via ambient pressure chemical vapor deposition method. The characterizations by spectroscopy and electron microscopy reveal that the as-grown MoS_2 film is mainly bilayer and trilayer with high quality. Back-gate field-effect transistor based on such MoS_2 The thin-film shows carrier mobility up to 3.4 cm-2V -1 s -1 and on / off ratio of 10-5. The large-area atomically thin MoS 2 prepared in this work has the potential for wide optoelectronic and photonic device applications.