Self-powered sensors are highly sought for wireless sensing applications in space exploration, industries, and environmental monitoring, etc. However, most current self-powered sensor technologies are based on the multiple energy conversion routine: energy collection, rectification, energy storage, and power management before it can be used for sensor systems, leading to exceptionally low energy utilization efficiency and very short periods of wireless sensing operation with majority of information lost. Here, we propose a triboelectric nanogenerator (TENG) based fully self-powered instantaneous and real-time wireless sensor system which does not contain electronic devices and microchips, but the passive components only. An innovative cylindrical capacitive-type liquid level sensor is also proposed and is then integrated into the wireless sensor system for monitoring liquid levels or identifying substance of the liquids. This sensor system can convert pulsed voltage output of the TENG into sinusoidal signal with a resonant frequency containing the sensing information and is transmitted to the receiver in distance in real-time. The maximum transmission distance of the sensor system could reach 1.5 m for a 10 cm diameter magnetic-core coil pair. The wireless sensor system exhibited excellent stability and excellent linearity with a sensitivity of 4.63 kHz/cm, and demonstrated its great application potential for the self-powered liquid level monitoring.