On-chip tunneling electron sources have wide potential applications in miniature vacuum electronic devices, and emission efficiency is one of their performance benchmarks. A cascade electron source (CES) based on series metal–insulator–metal horizontal tunneling junctions (HTJs) is proposed, where free electrons are additively extracted from each tunneling junction. A CES with $n$ HTJs shows a theoretical emission efficiency of approximately $η ( n )=1-( 1-η _0 )^n$ , with $η _0$ being the efficiency of a single tunneling junction. Experimentally, a CES with three Si–SiOx–Si tunneling junctions is demonstrated, achieving an emission efficiency of as high as 47.6%. This work provides a new way of realizing highly efficient on-chip tunneling electron sources.