Abstract: Volume density is an important component of object feature information, which can be used to quickly determine whether the inside of object changes or not. At present, the most commonly used method of detecting volume density is based on image reconstruction. First, using cone-beam projection data, we can acquire volume reconstruction, then we can obtain the volume density through the integration of object density function. This method is complicated and time consuming. It is difficult to achieve the real-time dynamical monitoring of volume density. This paper proposes a fast monitoring method of volume density based on cone-beam projections without image reconstruction. Experimental results show that this method has high real-time performance and accuracy to meet the actual needs of the dynamical monitoring, and thus can be used in applications of industrial detection.