The Rayleigh Brillouin Optical Time Domain Analysis system has the advantages of single source, single ended operation and non destruction. In order to solve the contradiction between the system signal-to-noise ratio and the spatial resolution, the coding technique was applied to the Rayleigh Brillouin Optical Time Domain Analysis system and a simplex code Rayleigh Brillouin Optical Time Domain Analysis sensing system based on the Avalanche Photo Diode detector was presented. The Rayleigh Brillouin Optical Time Domain Analysis system applied coding technology can improve the signal to noise ratio while maintaining the same spatial resolution. In the system, the random shot noise power was related to signal power, and the thermal noise power that was irrelevant to signal power mainly depended on the performance of the Avalanche Photo Diode detector. By analyzing the variance of current volatility which was relevant to two kinds of noise and Simplex code encoding and decoding rules, the mean square error of coding system was obtained, then the signal-to-noise ratio and coding gain formula were deduced. With the increase of coding length, the coding gain was also increased, then stabilized at a stable value, so the system had the optimal coding length. Finally, the optimal coding length formula was deduced. By using MATLAB it is verified that as the coding length increases, the coding gain tends to a stable value of 6.69 dB when the first-order Rayleigh scattering light power is 0.5 mW in the Rayleigh Brillouin Optical Time Domain Analysis system based on Simplex code, and the optimal code length was 63 bit.