A 780 nm on-chip integrated narrow linewidth semiconductor laser device is introduced in this paper, and this device is based on external cavity and master oscillator power amplifier structure (ECDL+MOPA). After introducing the main structure and operation principle of this laser diode, the design and simulation results of MOPA optical path, isolator parameters and pigtail optical path are described in details. Then the laser diode is realized and the main specification is indicated. After that, the laser linewidth test plan based on homodyne method and the detailed experiment setup are explained. This laser device is tested via the method introduced in this article, and it is proved that the output laser beam with linewidth smaller than 50 kHz@20 μs is realized. The related technology can be applied in atomic clock or other atomic interference fields.