Heavy ion beams with energy up to 1 AGeV can be provided by HIRFL-CSR in Lanzhou. By using the fragment separator RIBLL2, radioactive ion beams with energies of several hundred AMeV can be produced and transferred to the external target hall for experiments. To measure online γ-rays with energies below 5 MeV (center-of-mass frame), a CsI (Tl) detector array has been designed and a part of it with 768 CsI (Tl) crystals has been constructed. All the crystals with a cross section around 12 × 10 mm2 and a length between 15 and 17 cm have been produced and treated in the Institute of Modern Physics, Chinese Academy of Sciences. For reading out the scintillation light large area avalanche photodiodes (APDs) have been used. Based on ASIC chips and PXI bus a data acquisition system has also been developed. In the first test with a 60Co source an energy resolution of 7.2% (FWHM) has been achieved.