A real-time model of two-direction grinding force for external plunge grinding process has been built based on the research of grinding system dynamics equation. The two-direction grinding force can be measured on-line by a force cell sensor developed in this paper. And the grinding depth and grinding force, which depend on grinding time, were dynamic simulated by computer simulation. The experimental results are in good agreement with the simulation results, proved the real-time model’s exactitude. The real-time models showed in this paper not only can predict and evaluate grinding behave and quality, but also provide the precondition for grinding optimum, intelligential control, and virtual grinding research etc.