Self-powered wireless sensor systems are highly sought for the forthcoming Internet of Things era. However, most of the technologies take the route of energy harvesting, storage, and power regulation to power wireless sensor systems, which has a limited operation duration due to the low energy utilization efficiency of the multiple energy conversions involved. Here, we propose a triboelectric nanogenerator (TENG) based fully self-powered, instantaneous wireless sensor system which yet does not contain electronic devices and chips, but the passive components only. By integrating a capacitive sensor and an inductor coil with TENG, the pulse voltage output of the TENG is converted into a sinusoidal signal containing the sensing information with a resonant frequency and is transmitted to the receiver in distance wirelessly and continuously. A precise analytical model is developed for the capacitive sensor system with general implication; the oscillating signal generated by the model shows excellent agreement with experimental results. A capacitive humidity sensor is then utilized for sensing demonstration, showing that the maximum transmission distance of the sensor system is 50 and 90 cm for a 1 cm diameter magnetic-core coil pair and 20 cm diameter air-core coil pair, respectively. The wireless humidity sensor exhibits a sensitivity of 1.26 kHz/%RH, fast response speed, and excellent linearity, demonstrating its great application potential of the self-powered technology.