Commuting travel is the most important mode for urban residents.The vulnerability of complex commuting chain influences its normal running. This study constructs an index system for the vulnerability of complex commuter trip chain by 8 vulnerable points and 19 vulnerability observations. Based on reliability theory and BP neural network, a vulnerability measurement expression was derived. Through analyzing the trip investigation data of travellers in Xi'an, the generation and diffusion mechanism of vulnerability for complex commuting chain was obtained. The results show that the vulnerability of complex travel chain in Xi'an is relatively high（0.37）. And the innate vulnerability is mainly affected by trip time, personal attributes and trip costs, while the acquired vulnerability is mainly affected by traffic points, trip environment and transportation means, The vulnerability is mainly through traffic points, trip time, and trip costs to diffuse. The traffic control department should reduce the vulnerability of commuting travel chain by providing more comprehensive traffic information, dynamically monitoring the traffic system, and maintaining the stable operation of the traffic system . Key words: urban traffic; vulnerability; reliability theory; complex commuter trip chain; BP neural network

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