Abstract: Post mortem system is one of the key analysis tools for the accelerators to achieve high reliability and high availability. The precise time correlated data with high resolution that archived during the failure process is the base for the post mortem system. A software based post mortem system is designed, developed and implemented. By using this software based post mortem system in the accelerator of the China Spallation Neutron Source, most of the fast beam loss processes whose root cause could not be accurately analyzed with normal tools are well determined. This software based post mortem system is a general purpose analysis tool for most of the accelerators.