Abstract: Flexible piezoionic gels generate electrical signals through the asymmetric migration of anions and cations under mechanical deformation, offering advantages such as simple preparation, excellent mechanical properties, and facile signal generation, showing promising application prospects in smart sensing. However, these materials still face challenges including low output voltage and susceptibility to environmental interference. To address these limitations, current researches focus on combining ionic gels with functional groups, conductive polymers or piezoelectric polymers to develop flexible composite gels with enhanced piezoionic effects. Herein, from the perspective of composite material design, this review systematically elucidates how the design of functional groups on polymer molecules, polymer network, and multiphase composite structure modulate disparity in mobility rate of anion and cation within flexible composite gels. Key material design strategies for improving their piezoionic performances are summarized. Subsequently, recent advances of flexible composite gels in smart sensing such as information identification, biomedicine, and energy harvesting are introduced. Finally, current challenges of flexible piezoionic composite gels are outlined, including unclear microscopic mechanisms of piezoionic effects, limited output voltage, and insufficient system integration. Prospects for future research directions, such as mechanistic investigation, performance optimization, and the development of smart systems, are also discussed.