Abstract: The construction of Chinese Spallation Neutron Source (CSNS) laid the foundation for the development and application of neutron scattering technology General Purpose Powder Diffractometer (GPPD), which is one of the three spectrometers of CSNS in the first phase. The large area scintillator detector array for GPPD was developed and the key performance parameters of the detector has been tested on the BL20 of CSNS. The test results showed that the detection efficiency for 1.4 Å and 2.8 Å neutrons is (38.5±1.7)% and (56.1±1.1)% respectively. In accordance with simulation results, the detection efficiency is about (46.1±2.0)% for 2.0 Å neutrons. The position resolution of the detector is 4.0 mm×4.0 mm, the maximun counting rate is about 79 kHz and the count nonuniformity is about 19.6% and reduced to 14.9% after optimization. The performance of the detector unit can fulfill the physical requirements of GPPD.