The Safety Instrument System (SIS) is widely used in the fields of petrochemistry, railways, machinery, etc. to prevent accidents and improve safety. Based on the safety integrity level (SIL) demanded in the petrochemical industry, this paper proposes an architecture of a double-triple-channel redundant and fault-tolerant system (DTRFS) of SIS that uses double-triple-channel design to improve safety and availability. To research SIS's safety performance comprehensively, multiple failure modes and safety impact factors are considered when calculating a system's Probability of Failure on Demand (PFD), average Probability of Failing Safely (PFS) and average availability, while the Markov model and reliability block diagram are combined to construct a safety model. The results of MATLAB simulation show that the SIS with DTRFS architecture is able to reach the required SIL 3 in the petrochemical industry. Moreover, it has better safety performance than typical redundant and fault-tolerant systems. Also, the safety modelling method proposed in this paper is competent for SIS's reliability data and SIL calculation and offers clues for SIS design and improvement, which is highly valuable in engineering projects. (C) 2016 Elsevier Ltd. All rights reserved.