Triboelectric nanogenerators (TENGs) utilize the phenomena of contact electrification and electrostatic induction to harvest mechanical energy from the environment. A good match between the motion frequency and the circuit characteristic frequency is critical for the effective power generation of a TENG. However, most TENGs have a time-dependent inherent capacitance (TIC-TENG), which hinders an optimal design for efficient energy conversion. Here, we propose a novel structure of a TENG with a constant inherent capacitance (CIC-TENG) and a mathematical model is established to provide analytical expressions of key output parameters of the device, which gives numerical simulation results that are in good agreement with the experimentally obtained results. Figures of merit and an optimization strategy are also given as guidelines for the optimization of material selection, geometry design, etc. Furthermore, a disk-formed CIC-TENG (DCIC-TENG) with polarity-switched triboelectric pairs is constructed to harvest unidirectional mechanical energy continuously, achieving an output power density of 55 mW/m². The effects of the motion frequency, the number of electrodes and triboelectric pairs on the charge transfer efficiency of the DCIC-TENG are assessed and a preferred design strategy is given. Finally, the CIC-TENG demonstrates approximately two-fold advantages in power transfer efficiency over the TIC-TENG, and a DCIC-TENG-based self-powered anemometer was fabricated to measure wind speed in real time.