## 1 Introduction  
The energy consumption and greenhouse gas (GHG) emissions of China's transportation sector account for a large proportion of the total national energy consumption. Therefore, the transport sector must achieve low-carbon development transformation as soon as possible in the context of a global response to climate change [1, 2]. In this sense, the concept of a smart city has been applied to address the challenges, such as pollution, congestion, and lack of social infrastructure [3, 4, 5], to make cities more efficient, sustainable, equitable, and livable [6, 7, 8]. As an aspect of smart cities, the implementation of smart urban mobility [9, 10, 11], such as promoting public transportation, establishing bike-sharing systems, and restricting the use of cars, can reduce traffic congestion and GHG emissions [12, 13, 14]. As a new transport mode and part of the sharing economy, shared transportation is the innovation of the green development pattern, supporting sustainable urban transformations [15, 16]. Free-floating bike sharing (FFBS) is a part of shared transportation and a short-term bike rental service, which is connected to the internet through smart phones to help users park the dock-less bikes almost anywhere in the service area and enable smooth door-door transport without being limited by station infrastructure [17, 18, 19]. FFBS not only facilitates short-distance trips of three kilometers or less [20], but also expandsthe service radius of public transport by connecting to them, enhancing the possibility of multi-mobility [21; 22; 23]. The emergence of the FFBS allows the using right of bikes to be constantly switched, and bikes are used by different users at different times [24], which greatly improves the use efficiency of bikes. Two widely used patterns of FFBS are direct use and connection with public transport in a trip chain [25]. The emergence of FFBS provides new ideas for reducing greenhouse gas emissions and promoting urban sustainable travel [26; 27], which is of great significance to achieve the global environmental initiatives, including the United Nations' sustainable development goals [28] and the Paris Agreement. However, in the field of transportation, the existing research mainly focuses on the energy consumption and carbon emissions in the use phase of transportation modes. Moreover, fewer papers have calculated carbon emissions in the life cycle [29], let alone considering the trip chain. Life cycle assessment (LCA) is a more scientific and comprehensive evaluation method, which is usually applied to assess the impact of a system on the environment in the life cycle from cradle to grave. LCA has great flexibility with a systematic theory and standard during application. Therefore, from the perspective of the life cycle, comprehensive consideration of the four phases of production, use, operation, and disposal of FFBS can further verify whether FFBS really generates positive environmental benefits. The environmental benefits generated by FFBS are closely related to the modal shift, which is the difference between the GHG emission of using original transport modes and using other transport modes if FFBS was not available for the trip chain. Existing research on the modal shift is mainly based on simplified assumptions [30; 31], such as roughly setting the modal shift threshold using the trip distance without considering others. The modal shift structures influenced by FFBS are different in different usage patterns. For example, the mode of FFBS connecting public transport can better replace a car trip [32; 33]. Hence, it is necessary to analyze the different usage patterns of FFBS. In addition, the user's trip chain information cannot be obtained according to the historical trip data of FFBS. Therefore, this study analyzed the use characteristics of FFBS and the modal shift distribution under the influence of FFBS based on the trip chain through questionnaires, and evaluated the environmental benefits of FFBS.