On the basis of the characteristic and the inputing data format of preprocessing DDA (discontinuous deformation analysis) programs, a new program upgrading? AutoCAD platform was developed with Visual C++6.0nd AutoCAD ObjectARX developing tools. By using the Monte Carlo simulation method, a computer simulation of joints acco rding to the given distributions of joint sets was automatically generated on AutoCAD, and an inputting data file for DDA cutting program was also produced. The code makes the preprocessing DDA program more efficient and possesses practical engineering value.