In the field of soft strain sensors, piezo-transmittance based strain sensors, which detect strains by optical transmittance change, have promising advantages of fast response, high sensitivity, long-term stability, and negligible effect from environmental factors. However, they feature low sensor-to-sensor and in-sensor uniformity as well as unpredictable response and high stiffness. This study exploits the gap control of an auxetic-patterned elastomer to develop a piezo-transmittance based strain sensor. Gap opening mechanism in the negative Poisson’s ratio metamaterial with rotating square structures makes the sensor free from these limitations; thus, achieving a designable response and low stiffness. In addition, high sensor-to-sensor ((root-mean-square deviation (RSD) < 3.5%) and in-sensor (RSD < 5%) uniformities are achieved by uniform metal-deposited light-blocking film. Finally, the developed sensor has been integrated with a solar cell and Bluetooth Low Energy (BLE) 4.0 to afford a self-powered wireless strain sensing system that is successfully applied to structural health monitoring and human motion monitoring. A piezo-transmittance based strain sensor based on an auxetic structure with a gold-deposited elastomer is developed. Through this design, we accomplish high sensor-to-sensor and in-sensor uniformity. Furthermore, the gap opening mechanism allows an ultralow stiffness as well as a rational designability. The developed sensor is integrated into a self-powered wireless sensing system for structural health monitoring and human motion monitoring. Display omitted •A piezo-transmittance based self-powered strain sensor is developed.•High sensor uniformity is accomplished using a uniform mechanical metamaterial.•The gap opening mechanism allows an ultralow stiffness and rational designability.•A self-powered wireless sensing system using solar cell and BLE is developed.•The system is utilized for structural health monitoring and human motion monitoring.