In this work, conical cavities with a fixed top diameter and varied bottom diameter have been utilized to improve the signal intensity, signal-to-noise ratio (SNR), and signal stability of laser-induced breakdown spectroscopy (LIBS). It is observed that the postponement of the maximum enhancement of spectral intensity and SNR occurs abnormally with decreasing the bottom diameter due to the energy dissipation from the plasma to the cavity walls. Optimization of the cavity size indicates that the conical cavity is superior to the widely-used cylindrical cavity in improving the performance of LIBS. It is also found that the emission enhancement in the conical cavity with larger bottom diameters is attributed to the increase in plasma temperature and electron number density, but the enhancement in the conical cavity with smaller bottom diameters is ascribed to the growth of electron number density. For the first time, the exact efficiencies of conical cavities suppressing the total number density fluctuation are acquired to evaluate the performance of improving signal stability through analysis of the signal uncertainty composition. © 2022, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.