This paper presents a novel asymmetrical triboelectric nanogenerator (A-TENG) to produce, detect, and analyze contact electrification and electrostatic discharge (ESD) in the atmosphere. Thanks to the asymmetrical structures, the direct and continuous ESD phenomenon without any external electronic circuits is, for the first time, discovered by our experiments in A-TENG. Different from traditional contact-mode TENG, asymmetrical contact pairs introduce an unstable state, which causes a continuous surface charge increase and eventually the air breakdown. The ESD phenomena have been simultaneously detected and confirmed by a low-dark-current photoelectric detector. Four different steps have been summarized to describe irregular ESD transition processes before their stable state. At the same time, the frequency and efficiency of ESD have been generally regulated and controlled by systematically investigating several key influence factors (contact materials, contact pressure, tilted angle, surface morphology, etc.). This asymmetrical structure has proved TENG as powerful and real-time analytical equipment to explore fundamentals of contact electrification and ESD. Meanwhile, three necessary premises for ESD in TENG can be selectively avoided for the improvement of the stability of TENG.