Mixing characteristics in a plug-flow multi-stage (PFMS) anaerobic reactor and effects of shock loads in food waste treatment were examined In this study. Mixing in the PFMS reactor showed the typical characteristics of multi-stage reactor and was more effective than in a general type of longitudinal reactor mainly due to gas eruption from the gas hold-up spaces of columns. At steady state with 30 d of HRT, pH and COD were maintained stable, but pH and COD removal efficiency were quickly decreased under shock loads resulting from the unstability of the reactors under the shock loads. At steady states, acetic (HAc) and propionic (HPr) acids, were less than 4,000 mg/L for HAc and 2,000mg/L for HPr, respectively. However, HAc was increased up to 12,616 mg/L and HPr 17,569 mg/L when shock loads were applied. Gas production was ranged in 0.62~0.64 Nm3/kg VS removed and 1.6~1.7 Nm3/m3 digester-d. Methane production rates were rapidly decreased at shock loads, and remained unstable even after being returned to normal operating condition, indicating that the adverse effect of shock loads would not be short but lasted enough to lead system failure, and thus should be avoided. In particular, it was revealed that longer input interval at the same organic loads showed more detrimental on the reactor performance. Unlike TS, pH, organic fatty acids, organic removal efficiency and gas production rate were proven to be useful as indicators of shock loads.