If the mispredict happen, is there a mechanism to abort the instructions(have been fetched or decoded)?

For an in-order processor the mispredicted instructions are not executed and for an out-of-order one the any results that have been generated assuming the predicted path of the branch are discarded. Is there something more specific you are asking about? There's a Wikipedia article about out-of-order execution at

Out-of-order execution - Wikipedia, the free encyclopedia

For an in-order processor the mispredicted instructions are not executed and for an out-of-order one the any results that have been generated assuming the predicted path of the branch are discarded. Is there something more specific you are asking about? There's a Wikipedia article about out-of-order execution at

Out-of-order execution - Wikipedia, the free encyclopedia

If there is the mechanism, how does it work?

ARM will not give information about how the internals of the processors work in public forums - it's not information developers need to know in order to make effective use of the CPUs.

Regards,
Pete