A mass spectrum simulation system was developed. The simulated spectrum for a given target structure is computed based on the cleavage knowledge and statistical rules established and stored in pivot databases: cleavage rule knowledge, functional groups, small fragments and fragment-intensity relationships. These databases were constructed from correlation charts and statistical analysis of a large population of organic mass spectra using data mining techniques. Since 1980, several systems have been proposed for mass spectrum simulation, but at present there is no commercial software available. This shows the complexity and difficulties in the development of such a system. The reported mass spectral simulation system in this paper could be the first general software for organic chemistry use.