Objective:To study the effects of whole carbon fiber couch and virtual couch of VARIAN Linear Accelerator on radiotherapy dose of high energy X-ray.Methods:The solid water phantom was fixed at the center of treatment couch,to change the gantry angle,the different high-energy X-ray penetrated couch from different angle of gantry,and the penetration factor of X-ray was calculated by comparison and measurement.Radiotherapy plans with couch and non-couch were designed applying treatment planning system.Results:The penetration factor of each position,namely,thin,medium and thick,was at the lowest level,respectively,at gantry angle 110° for 6 MV and 15 MV X-ray following the change of incident gantry angle,the corresponding F was separately 0.9510,0.9496,0.9409 and 0.9668,0.9645,0.9600.The percentage difference between the measured value of these three positions of couch and the corresponding position of IGRT virtual couch reached the maximum level at gantry angle 160°,which were 2.79%,2.37%,1.81%and 1.15%,0.90%,0.53%,respectively.The variation trend of percentage difference between the measured value of these three positions of couch and the calculated value of non-virtual couch gradually decreased complying with the increase of couch thickness and X-ray energy.Also,value of PD reached the maximum level at about gantry angle 160° for 6 MV and 15 MV Xray,which were 2.06%,1.33%,0.50%and 0.50%,0.17%,-0.43%.Conclusion:Penetration factor decreases along with the couch thickness increase.The higher X-ray energy is,the bigger the penetration factor is.Under the condition of same X-ray energy,penetration factor gradually decreases with the couch thickness increase,meanwhile,penetration factor gradually increases along with the energy increase,that is to say,penetration factor for 15 MV X-ray is bigger than that for 6 MV X-ray,when penetrating the same medium.Under the condition of different X-ray energy,the variation trend of percentage difference between the measured value of three positions of treatment couch and the corresponding position of IGRT virtual couch is similar to penetration factor,namely,percentage difference gradually increases with the gantry angle increase.Under the condition of same X-ray energy,the percentage difference gradually decreases following the increase of couch thickness and X-ray energy.The variation trend of percentage difference between the measured value of three positions of couch and the calculated value of non-virtual couch gradually decreased complying with the increase of couch thickness and X-ray energy,however,the overall difference is lower than the calculation value of virtual couch.After designing radiation treatment planning with virtual couch using TPS,the measured dose is more close to the calculated dose.Therefore,it is necessary to use virtual couch when designing radiotherapy treatment plan in the clinical.