The Jiangmen Underground Neutrino Observatory (JUNO) is a neutrino ex- periment under construction in southern China. The main goal of the exper- iment is to determine the neutrino mass ordering and to measure three oscil- lation parameters with sub-percent precision. To reach these goals, JUNO is located about 52.5 km from two nuclear power plants and will detect electron antineutrinos from reactors through the inverse beta decay interaction. Fur- thermore, an unprecedented energy resolution of 3% at 1 MeV is required. The JUNO detector consists of 20 kt of liquid scintillator contained in an acrylic vessel instrumented by 17612 Large (20-inch) and 25 600 Small (3- inch) photomultiplier tubes (PMTs) with a total photocoverage around 78%. While the Large PMTs collect the great majority of the photon statsictics and thus drive the energy and vertex reconstruction of physics events, the Small PMTs operate in photon-counting mode and help calibrate the energy response of the Large PMTs. The Small PMT system front-end electronics (High Voltage, power sup- ply, readout and digitization, control) are installed under-water in waterproof tanks. Signals from physical events are readout and digitized in groups of 128 PMTs on a custom front-end board with 8 CATIROC 16-channel front- end ASIC chips in order to measure charge and time. This contribution will focus on the design and the performances of the front-end readout board as well as on the results from the integration tests with 128 3-inch PMTs and the full electronic chain.