With the advancement of flexible bioelectronics, developing highly elastic and breathable piezoelectric materials and devices that achieve conformal deformation, synchronous electromechanical coupling with the human body and high-fidelity collection of biological information remains a significant challenge. Here, a nanoconfinement self-assembly strategy is developed to prepare elastic phenylalanine dipeptide (FF) crystal fibers, in which FF crystals form a unique Mortise-Tenon structure with oriented styrene-block-butadiene-block-styrene molecular beams and thereby obtain elasticity (≈1200%), flexibility (Young’s modulus: 0.409 ± 0.031 MPa), piezoelectricity (macroscopic d33: 10.025 ± 0.33 pC N-1), breathability, and physical stability. Furthermore, elastic FF crystal fibers are used to develop a flexible human physiological movement sensing system by integrating Ga–In alloy coating and wireless electronic transmission components. The system can undergo conformal deformation with human skin and achieve high-fidelity capture of biological information originating from human body motions to prevent diseases (such as Parkinson’s disease). In addition, this system also displays superior sensitivity and accuracy in detecting subtle pressure changes in vivo during heartbeats, respiration, and diaphragm movement. Therefore, elastic FF crystal fibers hold great potential for developing new flexible electromechanical sensors that are capable of conformal deformation with the human body, enabling precision medical diagnosis and efficient energy harvesting. A schematic illustration depicting the utilization of styrene-block-butadiene-block-styrene (SBS) fibers as a self-assembly nanoconfinement carrier for phenylalanine dipeptide (FF) has been provided, showcasing the formation mechanism of elastic FF crystal fibers featuring a distinctive Mortise-Tenon structure.