The work aims to solve the problem that the traditional hard plate capacitive liquid level sensor cannot be strictly attached to the curved container. A flexible plate capacitive liquid level sensor which can be closely attached to the curved container was designed according to the basic principle of stray capacitance. The mathematical relationship between the liquid level and the capacitance was established, and the corresponding soft and hard design was completed. Through the I2C communication between STM32 and the flexible plate capacitive liquid level sensor, the online water level detection of the liquid level sensor, and the performance test experiment of sensor were carried out, including performance indicators such as linearity, repeatability and hysteresis characteristics. A flow measurement method based on the sensor was proposed. The sensor worked stably with good linearity. Its repeatability error was 2.70%, and the hysteresis characteristic parameters in the effective measurement range were all less than 1.69%. The flexible plate capacitive liquid level sensor is easy to be installed and used, has a good fit with the outer wall of the container, and the measurement result is not limited by the horizontal cross-sectional area of the container and the composition of the liquid to be measured. The continuously changing liquid level can be monitored in real time. The sensor made in this article can be used on the container to be measured with a diameter of 20 mm and smaller, and can be used for micro flow measurement.