The main point of the hydrodynamic model of the electromagnetic field (Deng Ximing & Fang Honglie 1979, 1980 is that the motion of the electromagnetic field can be divided into two parts: orbital motion and intrinsic motion. This paper defines an intrinsic angular momentum deduced from the intrinsic motion and a related / (imaginary number) operator, whose basic properties are discussed. In addition, the conservation property of the intrinsic angular momentum and the relation between it and the spin angular momentum of the electromagnetic field are described.