Here is a transmitter for an oxygen-band differential absorption lidar that is tried for atmospheric pressure detection experiment. The laser transmitter is based on the structure of a seed-injected optical parametric oscillator and an optical parametric amplifier. As a slave oscillator, a ring cavity KTP optical parametric oscillator is used. As the master oscillator, is a continuous wave external cavity diode laser. Operating wavelength of the continuous wave external cavity diode laser was stabilized, by a PID (Proportional-Integral-Derivative) servo control loop composed of a high-precision wavelength meter. A continuous wave seed laser is injected into the optical parametric oscillator cavity, and the cavity length of the OPO (Optical Parametric Oscillator) resonator is locked by the “Ramp-Hold-Fire” technique. This laser transmitter has been proven to have the following properties: high optical frequency stability (30 MHz/rms), narrow linewidth (Fourier transform limited), and high pulse energy (≥45 mJ),which can be maintained during operation. The transmitter system operates with a single longitudinal mode, making it possible for differential absorption lidar to detect narrowband backscattering signals. Such systems therefore have the developing potential to accurately measure atmospheric pressure.