A novel type broad beam cool cathode ion source was successfully developed for optical thin film growth technique by ion beam assisted deposition. The structure, discharge process and working properties were introduced in this paper. The ion energy and its distribution were investigated by a 5-grid tester. The results indicates that the lowest ion energy in beam increases with the increase of the extraction voltage, whereas decreases with the increase of the pressure. The peak position of the ion energy distribution function shifts to the low energy with the increase of the pressure, whereas to the high energy with the increase of the extraction voltage. The average ion energy is about 600～1 600 eV at the extraction voltage of 200～1 200 V, and the initial ion energy is 430～480 eV. The ion energy and energy distribution can be controlled effectively by regulating the working parameters of the ion source during the film deposition process.