A flexible fiber strain sensor with multi-layer hollow structure is prepared by coaxial wet spinning technology.The outer layer is an elastic substrate layer made of pure Ecoflex material

and the inner layer is a conductive layer made of carbon black/polydimethylsiloxane（ CB）/PDMS composite material.The hollow structure is caused by volatilization of CHCl3 when the CB/PDMS/CHCl3 mixture is heated.Simultaneously

the CB/PDMS composite coating produces a cracked structure

which enhances the gauge factor（ GF） of the hollow fiber

the inner coating shows obvious disconnection-recovery phenomenon with stretching.Experiments show that when the volume ratio of added CHCl3 to PDMS is 3: 1

the CB-Ecoflex fiber has the highest sensitivity.The GF value is 7.14

which is compared with non-cracked CB-Ecoflex fiber improved by about six times.Research on human motion monitoring is carried out

and it is found that CB-Ecoflex fiber show good response characteristics to human motion.The flexible strain sensor prepared by this method provides a new path for the preparation of wearable electronic devices.