The use of triboelectric nanogenerators (TENG) is a highly effective technology for harvesting ambient mechanical energy to produce electricity. In this work, a theoretical model of rotary freestanding TENG with grating structure is constructed, including the conductor-to-dielectric and dielectric-to-dielectric categories. The finite element simulations are performed to capture the fundamental physics of rotary freestanding TENG. Based on the simulations and derivations of approximate analytical equations, the real-time output characteristics of TENG with arbitrary load resistance are calculated. Furthermore, the influences of structural parameters of TENG and rotation rate on the output performance are investigated. The theory presented here facilitates a deep understanding of the working mechanism of rotary freestanding TENG and provides useful guidance for designing high performance TENG for energy harvesting applications.