Abstract: To study critical collapse at rear surface of concrete targets under internal explosion, the collapse on the rear surface of concrete targets with finite thickness under internal explosion was studied, and the engineering formula of the dimensionless critical collapse thickness was obtained. The result shows that, when the density of charge is fixed, the critical collapse thickness increases non-linearly with the increase of the diameter or the length-to-diameter ratio of charge, and has a limit value. In addition, it is found that there exists the second kind of similarity of the critical collapse thickness in the diameter of charge.