The invention provides a protocol data unit detection method and device so as to solve the problem that service interruption is caused by reconstruction or switchover. The protocol data unit detection method comprises the steps as follows: a protocol data unit is received, and the existence that the protocol data unit is outside a reception window can be detected; if the cache outside the window is empty and the protocol data unit submitted in the last time is outside the reception window, the protocol data unit is processed through a first submission, wherein the cache outside the window is used for storing the protocol data unit outside the reception window; if the cache outside the window is not empty or the cache outside the window is empty and the protocol data unit submitted in the last time is not outside the reception window, the protocol data unit is put in the cache outside the window; and the protocol data unit in the cache outside the window can be processed through the first submission when the cache is full.