Considering that for traditional single-pulse Brillouin Optical Time Domain Reflectometer(BOTDR) system, Brillouin signal is weak and the performance improvement is limited, a Golay pulse coding BOTDR system with Avalanche Photo Diode(APD) detector and local-heterodyne detection was proposed. The coding and decoding principle of Golay code in applying to the system and the principle of local-heterodyne detection were analyzed. The restriction of Stimulated Brillouin Scattering threshold in optical fiber on the coding length of the system was discussed. The mathematical expression of signal-to-noise ratio was deduced and the relationship of signal-to-noise ratio with APD multiplication factor and coding length was studied. The expressions of the optimal multiplication factor of APD and the optimal coding length were obtained, respectively. The MATLAB simulation results show that the optimal value of APD multiplication factor and the optimum coding length exist for this specific system with a bandwidth of the APD detector of 500 MHz, and the peak power and pulse width of 50 mW and 100 ns. The determination of optimum coding length of the system depends on not only the shot noise and thermal noise power, but also the Stimulated Brillouin Scattering threshold of optical fiber. With the APD optimal multiplication factor of 5 and the optimal coding length of 128 bits, the signal-to-noise ratio at the end of 25 km ending fiber increased by 26.42 dB compared to that of traditional single-pulse system, and temperature and strain resolution of 1.60 ℃ and 35.48 are achieved, respectively.