With the rapid development of optical fiber technology and optical communication technology,distributed optical fiber sensor as the fusion of the two derivative types sensing technology has graduall y stepped into all aspects of the national economy as well as people′s livelihood .The principles of Brillouin optical time domain analysis (BOTDA) system based on pulse coding are analyzed. The system launches optical pulse sequence produced by encoded pulse modulation to the sensing fiber ,and retrieves the single-pulse response with a data processing program at the receiving end of th e system,thereby enhancing the signal-to-noise ratio (SNR) and the temp erature or strain resolution of BOTDA system.In order to improve the SNR of BOTDA system,the paper proposes a new coding method combini ng CCPONS code with Walsh code.The coding and decoding principles applying to the B OTDA system are analyzed.The signal-to-noise improvement ratio (SNIR) of the BOTDA system is derived after inducing CCPONS code,Walsh code and combined code,and the improvement of the SNR of the three codes is analyzed.The results show that the improvement of the SNR of the BOTDA system after inducing the combined code is the best.