Microcavities constructed from materials with a second-order nonlinear coefficient have enabled efficient second-harmonic (SH) generation at a low power level. However, it is still technically challenging to realize double resonance with large nonlinear modal overlap in a microcavity. Here, we propose a design for a robust, tunable, and easy coupling double-resonance SH generation based on the combination of a newly developed fiber-based Fabry–Perot microcavity and a sandwich structure, whose numerical SH conversion efficiency is up to 3000% W^-1. This proposal provides a feasible way to construct ultra-efficient nonlinear devices for generation of classical and quantum light sources.