Triboelectric nanogenerator is an emerging technology that can convert low-frequency and irregular mechanical energy into electricity, which has broad applications in the fields of distributed energy harvesting, self-powered sensing, and the Internet of Things. However, it is still a challenge to achieve high electrical energy density owing to the limited surface tribo-charge density. Here, we report a three-dimensional fractal structured nanogenerator (FSNG), which can much more efficiently collect the triboelectrification-induced space distributed Maxwell’s displacement current, and hence largely improve the electrical energy density. The output charge density, power density and energy density of FSNG break the record and reach up to 8 mC/m2, 2.18 MW/m2 and 0.39 J∙m-2∙cycle-1, respectively, which can be further improved by optimizing the fractal unit. In addition, the equivalent fractal circuit models for FSNG with the same functionality are developed for deep understanding the FSNG. This study not only creates a new record of high electrical output of nanogenerators, but also pave a more highly efficient way for mechanical energy scavenging towards practical applications.