Abstract: The explosion reason of a bladder type accumulator was analyzed by means of chemical compositions analysis, mechanical test, metallographic examination, macro and micro fracture analysis, bursting pressure estimating and so on. The results show that the bursting pressure of the accumulator was far higher than the design pressure and this is a typical chemical explosion. The main cause for the accident was that oxygen were filled in the bladder by mistake, which resulted in the internal combustion and explosion of the accumulator.