All-fiber vital signs monitoring based on long-modal-interference (LMI) in multimode fiber (MMF) is proposed and investigated theoretically and experimentally. The LMI-MMF is simply formed by splicing of a two-meter-long MMF and two single mode fibers (SMFs) with core-offsets at splicing joints. It is worth mentioning that all fibers that used are sheathed with 900-μm protection sleeve. Core-offset dependency of vital signs monitoring performance is specifically analyzed. Experiments are carried out on subjects with weight from 56 kg to 103 kg without further optimization or modification. Compared with commercial piezoceramic-based respiratory sensor and SpO2-based heartbeat sensor, the proposed non-wearable all-fiber vital signs monitoring sensor based on LMI-MMF shows great consistence with high Person's correlation coefficient. It is the first time to put forward the LMI-MMF-based fiber sensor and apply it to realize the non-wearable vital signs monitoring. Specific superiorities of low cost, real time and non-intrusiveness make it potentially competitive in household healthcare and other medical fields. © 2020 Elsevier B.V.