Dense granular systems flowing through bottleneck is prone to spontaneously form clogging. Clogging is common in various industries, which makes the process requirements of continuous production might not be accomplished. Therefore, it is very important to clarify the dynamic characteristics and clogging mechanism for the optimal design of anti-clogging system. In this work, clogging mechanisms are examined experimentally and numerically. The results indicate that there is indeed a region with larger fluctuation of kinematic characteristics near the outlet, which we called "dynamic arch". But, the fluctuation characteristics are not always related to the formation of clogging which can help to distinguish two clogging mechanisms, including "collision clogging" and "jamming". Further, we also illustrated microscopic mechanisms of clogging and discussed fully the different clogging mechanisms. The definite clogging mechanisms lay foundations for a better designs of devices for preventing formation of the clogging. (C) 2020 Elsevier B.V. All rights reserved.