Flexible capacitive strain sensors, with the characteristics of high flexibility, light weight, low power consumption, and easy integration, have important application values in fields such as health monitoring, human-computer interaction, robotics, and smart textiles. At the present stage, it is difficult for flexible capacitive strain sensors to achieve a linear response of the capacitance signal under wide strain, and the application characteristics in specific scenarios have not received due attention. In response to this, this study proposes a capacitive elastic strain sensor using liquid metal as the electrode layer and ecological flexible rubber as the dielectric layer for respiratory monitoring. Through experimental tests, it is found that the gauge factor of this sensor can reach 1.4 within the strain range of 0%~120%, and the minimum resolution is 0.01 mm. After 1 million stretching cycles of 3%, it can still output the capacitance signal. It can be used for long-term monitoring of respiratory signals with tiny deformations, and it has good washability and ethanol disinfection resistance. It can judge the human body′s movement state and respiratory state according to the changes in respiration, providing a new development direction for intelligent healthcare.