Abstract: After analyzing the effects of the actual design procedure and the substantial influence factors on stresses in high-temperature pipeline, it is found that the thermal stresses of a pipeline are mainly dominated by the routing trend of the piping system and the actual length of each pipe segment. Therefore, this paper puts forward the concept of pipe length ratio. Using force method to analyze the effects of lengths of the pipe segments on thermal stresses in several typical piping patterns, following results have been found: (1) The larger the length ratio of adjacent pipe sections is, the greater the maximum value of thermal stress can be obtained corresponding to various piping arrangements; (2) taking P91 pipeline as an example, the minimum allowable pipe section length of a pipe system is obtained for the given pipe sizes and operation data; (3) the maximum length ratio of pipe system is obtained through determining the minimum length of a pipe system; (4) the shortest allowable pipe length of a pipe system approaches to an asymptotic value with the increase of the number of pipe sections; (5) when the minimum length of a pipe system is determined, the allowable maximum length ratios approaches to an asymptotic value with the increase of bends.