The importance of diesel injection system simulation as a design tool is widely recognized and various models have been developed. But most of the simulations are implemented with digital computers. A new simulation model using digital-analog hybrid computer has been developed. The objective of this simulation is to optimize the injection system design with minimum time by interactive method. The hybrid computer meets this requirement. One of the main features of this model is the introduction of a simple equation about the bulk modulus of elasticity of fuel oil. It is expressed as a function of the oil pressure and the volume ratio of bubble to fluid. This paper discribes the detail of the modeling and the programming of this simulation for hybrid computer, comparison between calculation and experiments and some examples of application to the actual diesel injection system.