Organic–inorganic nanocomposites with superior dielectric energy density are highly sought after for high-power electronics and pulsed power systems, and interfacial engineering turns out to be one of the most successful strategies to improve their breakdown strength. However, a microscopic mechanism responsible for such improvement, thought to be closely related to local field fluctuation in the nanocomposites, has never been directly demonstrated experimentally. Here, we develop a powerful yet readily applicable in situ technique to evaluate the fluctuation of electric field in dielectrics, revealing reduced field fluctuation in interfacial engineered nanocomposites that clearly correlates with its enhanced breakdown strength. This work, thus, validates field fluctuation-based breakdown criterion of nanocomposite proposed more than one decade ago and provides further support to improve organic–inorganic nanocomposites for high density dielectric energy storage.