Abstract: Foam material of low density is the preferred medium for X-ray radiation transport experiment, and the low energy X-ray radioscopy is the important technology for foam density measuring. The every error origin in the process of foam density measuring was investigated. According to the international general method, the component uncertainties were combined and expanded respectively to obtain the uncertainty evaluation. The results indicate that, the horizontal resolution is over 10 μm, the measuring uncertainty of density is under 4.7%, at the same time, it has applied in SGⅡ physical experiment successfully, and has acquired satisfactory effect.