Observations show that there are proton spectral
``breaks'' with energy $E_{\rm br}$ at 1-10MeV in some large solar
energetic particle (SEP) events. Generally, single CME-driven shock
applying diffusive acceleration mechanism would just predict a
single power-law energy spectrum. This work discusses the difference
of the energy spectra between a single shock and the converging
double-shock. We apply a single shock and a converging double-shock
models to the 2006 Dec 13 SEP event using particle simulation
method, respectively. As results, we find that a single shock model
just produce an energy spectrum with a single-power law, but a
double converging-shock model can produce a ``broken'' energy
spectrum splitting to double power-law, which is consistent with the
observed energy spectrum by spacecraft.