The tremendous developments of power systems bring new challenge to the simulation technology and conventional transient simulation cannot meet the analytical needs of practical large-scale power systems. Combining the advantages of electromagnetic and electromechanical transient simulations， experts and scholars have proposed the hybrid simulation technology. This paper gives an overview on the research and development of the hybrid simulation at home and abroad. Then two fundamental requirements for engineering applications are specified and several essential difficulties are described， including the system segmentation scheme， the boundary condition for the calculation of one side and equivalence of the other side， and the interactive order of two sides during the system fault. Based on the analysis of these difficulties， a forecast and correction method is proposed， with its feasibility expounded. Finally， the paper makes a simple comment on the development trends of the hybrid simulation.