High sensitivity flexible strain sensors that can perceive and respond to strain stimulation are an essential part of wearable smart devices. Yet, the detection of subtle strains on the human body and external stimulation suffers from low sensitivity and slow response of many existing sensors. Here, an ultrasensitive flexible optical sensor based on microfiber coupler encapsulated by Polydimethylsiloxane (PDMS) has been proposed to realize weak strain detection and fast response. Benefiting from dramatic change of coupling ratio (CR) of the microfiber coupler under weak strain, this flexible strain sensor exhibits ultrahigh strain sensitivity (gauge factor, GF=900), low detection limit of 0.001%, ultrafast response time (< 0.167 ms), and superior durability and stability (>10 000 cycles). Synchronously, the simultaneous detection of CR and loss parameters not only realizes the 0.001% low detection limit, but also expands the sensing range to 0.45%. This highly sensitive microfiber coupler sensor has been demonstrated to capture and recognize respiration, broadband sound signals and pulse waves at different sites of human body in real time. Moreover, based on frequency division multiplexing technology, the wrist pulse and human voice could be detected and demodulated simultaneously. This flexible photonics strain sensor could be useful in the development of convenient and wearable healthcare devices. Corresponding author(s) Email:  [email protected]； [email protected];