Abstract: Stem expansion and contraction are closely related to the plant water status. It is of great significance to real-time monitor the stem expansion and contraction, in order to improve the utilization rate of agricultural water resources in irrigation systems. Therefore, the sensors have been mainly used to monitor the expansion and contraction of plant stems at present. The linear micro-displacement can be measured to assess the plant water status. However, it is still limited to the high cost and inconvenient installation. In this study, a flexible and wearable sensor was designed and developed using the piezoresistive effect. A flexible pressure electrode was used as the sensing element to monitor the pressure variation, which was attached to the surface of plant stems. Different from the linear micro-displacement sensor, the developed sensor was used to assess the pressure variation caused by the expansion and contraction of the stem. The pressure signal detection and data collection were also designed to convert the pressure into electrical signals for storage. A series of experiments were conducted to evaluate the performance of the pressure sensor. Firstly, the performance test and sensor calibration were conducted under laboratory conditions. Then, the pressure sensor was installed on a tomato stem in a greenhouse, in order to observe the pressure variation. A comparison was made with the linear micro-displacement sensor. Finally, the variations in the expansion and contraction of tomato stems were observed under sufficient irrigation and water deficit using the two types of sensors. The results show that the average relative change rate of the stability test for flexible pressure sensors was 0.109%; There was negligible output variation caused by bending; The determination coefficient of calibration was greater than 0.99, and the most suitable working range was 2-100 kPa; The determination coefficient of the measurements between the two types of sensors was 0.955 1, compared with the linear micro displacement sensors. The greenhouse experiments show that the determination coefficients of the measurements using the two types of sensors were 0.767 2 and 0.851 9, respectively, for full irrigation and deficient irrigation. The flexible and wearable pressure sensor can be expected to monitor the variations in the expansion and contraction of tomato stems. The water deficit stress of the tomato can also be diagnosed as well. The pressure sensor achieved better performance during the experiment, such as the low cost and easy installation, compared with the linear micro-displacement sensors.