Objective To study the method of using phantom test for daily quality assurance of on board image(OBI) system. Methods The routine procedures of radiotherapy, including CT simulation, planning,setup,cone beam CT(CBCT) scan and kilovolt X-ray orthogonal film were carried out on a head phantom. The procedures repeated once a day in the following 10 days. The geometric errors of the phantom were recorded. Results The geometric errors of the phantom by CBCT were [0.06±0.11] cm, [0.03±0.05] cm, [0.07±0.07] cm and [0.03±0.10] cm in longitudinal, vertical, lateral and rotation directions, respectively. The geometric errors of the phantom by kilovolt X-ray orthogonal film were [ 0.04±0.10] cm, [0.03±0.05] cm, [0.08±0.06] cm and [0.05±0.05] cm, respectively. The differences of geometric errors of the phantom by CBCT and kilovolt X-ray orthogonal film were not significant(t = 0.44,P=0.667 in longitudinal direction ; t=0.00, P=1.060 in vertical direction ; t=0.34, P=0.735 in lateral direction; t=0.58,P=0.568 in rotation direction). Conclusions The OBI system of our accelerator is reliable and at excellent performance status. The method by using phantom test for the daily quality assurance of OBI system is easy and reliable.