The source separation and decentralised treatment technology for domestic wastewater is an important method to improve sewage treatment rates and water reuse. The Living Machine (LM) system is a widely applied decentralised domestic wastewater treatment technology. In order to enhance the black water (toilet water) treatment performance, an underwater lamp was placed in the open aerobic reactor of the LM system, and treatment efficiency and micro-ecosystem changes with different light intensities were studied in this research. Compared to the control reactor (C), removal efficiencies of ammonia, total nitrogen (TN), and total phosphorus (TP) were increased from 22.07%, 16.03%, and 13.81% to 42.20%, 22.82%, and 17.20%, respectively, in the underwater light reactor (L) with an average intensity of 2083.00 LX, and COD removal was not obviously effected. The species and numbers of algae and the species of zooplankton and other aquatic animals were increased sharply in L, but their number did not fit a linear relation with light intensity. The bacteria group was prominently changed in L, whereas Malikia_spinosa, Emticicia_sediminis, and Rhodocyclaceae_bacterium_MORI13 were increased, and Rhodocyclaceae_bacterium_ICHIDE18 was obviously decreased. Although underwater illumination enhanced the wastewater reactor performance of the LM open aerobic reactor, further research is needed to learn more about the process optimization and relative mechanism. Display omitted •N and P removal and DO level increased by installing underwater lamp.•Algae and aquatic animal's species richness increased by underwater illumination.•Microbial communities prominently changed in underwater illuminated reactors.