Hardware-in-the-loop simulation combines the merits of digital simulation and physical simulation， and is a new development direction of simulation technology. The interface algorithm is a key issue in the hardware-in-the-loop simulation. The structure of the hardware-in-the-loop simulation system is analyzed， and the time frames of series flow and parallel flow are compared. Five different interface algorithms are described， and the uniform expression of four algorithms is derived. At last， the discrete dynamic model of the hardware-in-the-loop simulation system is constructed to provide the foundation for theoretical analysis.