According to the latest physics design of the China Fusion Engineering Test Reactor (CFETR), two neutral beam injectors (NBIs), which deliver a total of 40 MW in not less than 3600 s with 1 MeV D0, are demanded to support current drive and plasma rotation. To minimize the risks and time to provide the CFETR with reliable NBIs, a negative NBI test facility will be developed at the Institute of Plasma Physics, Chinese Academy of Science. Its mission is to understand the characteristics of the RF driven ion source and negative ion generation and extraction and to improve RF efficiency and beam quality. In order to achieve this goal, a set of diagnostic tools will be used in this test facility. For source diagnostics, optical emission spectroscopy, cavity ring-down spectroscopy, laser absorption  pectroscopy, and electrostatic probes are planned to be used. Beam emission spectroscopy, W-wire calorimeters, 1D carbon fiber composite diagnostic calorimeters, beam dump with thermocouples, and water-flow calorimetry are used to assess the beam properties. The design of the diagnostic system is presented.