In order to evaluate effectiveness of target camouflage objectively and quantitatively, a model of similarity degree is established based on a descriptor of attention focus derived from the human visual attention mechanism (HVAM). Locations and shifts of the attention focus are searched and measured using the HVAM model. The descriptors of attention focus are extracted based on their characteristic scale levels. The model of characteristic similarity degree is constructed on the descriptor, which can quantitatively evaluate the degree of amalgamation of target and surrounding background in brightness, color and space distribution. The relation between characteristic similarity degree and target detection probability is given. Applications show that this model has advantages such as high determinability and operability.