A carbon nanotube(CNT) thin film field emission cathode was prepared by drop casting method,and its field emission characterisitic was studied by ultra violet photoelectron emission spectroscopy and Raman spectroscopy.The results indicated that the field emission curves of the CNT thin films obeyed the Fowler-Nordheim(FN) theory.The minimum turn-on field and the threshold field were 3.33 MV/m and 5.44 MV/m,respectively.The field enhancement factor was found to be around 103.The ultraviolet photoelectron emission spectral analysis showed that the low-energy cut-off edge of the CNTs thin film gradually decreases as the applied bias increases,which indicated that the applied bias has pull down the effective emission barrier greatly so that electrons can emit at smaller electrical field without geometric enhancement.The reduction of effective emission barrier is explained by the contact effect from the CNTs/Ni interface.In addition,the bias drop on the thin film also leads to the reduction of the surface barrier,which is particularly obvious in the unpurified CNTs thin film,and it is related to the residue of amorphous carbon(a-C) in the thin film.