The spatial resolution of traditional Brillouin optical time-domain analysis (BOTDA) sensor is often unable to achieve the spatial resolution within 1 m due to the influence of 10 ns acoustic phonon lifetime in optical fibers. The differential pulse pair (DPP) technique can avoid this limitation and obtain higher spatial resolution. However, the traditional DPP technique has problems, such as long measurement time, high difficulty in synchronization, and low signal-to-noise ratio. In this paper, we propose a coded DPP-BOTDA system based on the Brillouin gain-loss effect. the pump pulse light at Stokes frequency and anti-Stokes frequency is synchronously injected into the optical fiber, and the Brillouin gain loss effect of the scattered light is used to differentiate on the optical path to solve the synchronization problem between signals, and the measurement time is half of the traditional DPP method. The influence of the gain characteristics of an erbium-doped fiber amplifier in the sensing system on the decoding results of pulse sequences is analyzed, and the coding gain under the condition of uneven gain is calculated theoretically. The experimental results show that the system can obtain a spatial resolution of 50 cm with a signal-to-noise ratio improvement of 3 dB compared with the traditional single-pulse DPP-BOTDA system.