The theory of system simulation is used to calculate an estimation of project’s completion period, based on which the project procedures are modeled. The key approach is to use Monte Carlo simulation. With the power of Java multithreading technique, a parallel grid calculation is applied to simulate an example and optimize it. Computing result and time record show that, not only this approach can work out the project schedule estimation with distribution and a certain critical path, but also shorten the simulation time and increase the efficiency.