Environmentally induced sensor temperature fluctuations can distort the outputs of a sensor, reducing their stability during long-term health monitoring. Here, a passive isothermal flexible sensor is proposed by using hierarchical cellulose aerogel (HCA) as the top tribonegative layer, which allows the sensor to adapt dynamic thermal environments through both radiative cooling and heat insulation. The radiative cooling effect can cool down the temperatures of a sensor in summer, while the hollow microfibers in HCA provide ultralow thermal conductivity to reduce internal heat loss in winter. The prepared passive isothermal sensor is capable of maintaining the rated working temperature over an extensive temperature range of 0−100 °C, demonstrating for gripping hot and cold objects. While monitoring human movements under direct sunlight, the temperature of a conventional sensor rose by 12.3 °C, whereas the sensor experienced an increase of only 0.3 °C. Therefore, this work presents a promising strategy for adapting to environments, enabling wearable electronics to function effectively in dynamic thermal conditions.