Abstract: The thermal neutron reference radiation field at thermal column of the heavy water reactor of China Institute of Atomic Energy (CIAE) has been established. Its energy spectrum has been measured by use of a chopper. The absolute fluence rate has been acquired by using both gold activation technique and a 235 U ionization chamber.The results are respectively 1.14 ×10 6(1±1.2 %) cm -2 ·s -1 and 1.15×10 6(1±2.2 %) cm -2 ·s -1. The neutron fluence distribution along x,y,z directions, its cadmium ratio and the gamma ray dose of the beam have also been measured as well. Therein, the cadmium ratio is 1.1×10 4(1±10 %) and the gamma ray doses are 5 mGy·h -1 (40 cm to the hole of reactor) and 0.9 mGy·h -1 (100 cm), respectively.