Recent remarkable advances in flexible and wearable functional electronics have been achieved due to their advantages of lightweight, convenience to detect complex measured objects, and flexibility, which makes electronic equipment develop from a rigid system to a flexible system. Flexible materials toward wearable devices present a huge market prospect, which demonstrate promise in healthcare, human motion detection, biomedicine, environmental monitoring, and other fields. Unique physical and chemical properties are the critical characteristics of flexible wearable sensors, and the huge surface-to-volume ratio facilitates carbon nanomaterials to be used in various flexible sensors. This review comprehensively introduces the structure, properties, and preparation of carbon nanomaterials like carbon nanotubes, graphene, and MXene. The sensing mechanism of the sensor is outlined, and flexible sensors based on carbon nanomaterials in detecting targets such as pressure, temperature, and gases are proposed. Enhancement of conductivity, sensing factor, and sensitivity of carbon nanomaterials by modifying their structure or preparing composites is presented in detail. Finally, this paper outlines the challenges associated with implementing carbon nanomaterials within the field of flexible wearable sensors and future development.