Triboelectric nanogenerators (TENGs) that convert mechanical energy into electricity have attracted tremendous attention and kinds of strategies are proposed to optimize the electrical performance of TENG. In this work, we fabricated a TENG based on a sandwich-structured polyimide (PI)/boron nitride nanosheet (BNNS)/PI nanocomposite film (named PBP). The PBP-TENG exhibits an optimized open-circuit voltage of 65.9 V and a short-circuit current of 4.5 μA. When the load resistance was 10 MΩ, the introduction of a BNNS interlayer raised the power density of the TENG up to 21.4 μW/cm2, which is 15 times higher than that of TENG without a BNNS interlayer (PP-TENG). It is found that the relatively high dielectric constant of BNNS leads to enhancing the output performance of the TENG. More importantly, PBP-TENG also maintains excellent output even in a high-humidity environment.