Second-order nonlinear optical processes lie at the heart of many applications in both classical and quantum regimes1–3. Inversion symmetry, however, rules out the second-order nonlinear electric-dipole response1,4,5 in materials widely adopted in integrated photonics (for example, SiO2, Si and Si3N4). Here, we report nonlinear optics induced by symmetry breaking6–10 at the surface of an ultrahigh-Q silica microcavity under a sub-milliwatt continuous-wave pump. By dynamically coordinating the double-resonance phase matching, a second harmonic is achieved with an unprecedented conversion efficiency of 0.049% W−1, 14 orders of magnitude higher than that of the non-enhancement case11. In addition, the nonlinear effect from the intrinsic symmetry breaking at the surface8,12 can be identified unambiguously, with guided control of the pump polarization and the recognition of the second-harmonic mode distribution. This work not only extends the emission frequency range of silica photonic devices, but also lays the groundwork for applications in ultra-sensitive surface analysis.