Hello SeetXuXiang,
This Issue is mainly caused when the connection between your Computer and the Server can not be established properly. In technical terms, Data in the form of Data Packets sent by your Internet Service Provider from your Computer to the Server gets lost in the Network Layer ( route used by Data Packets to travel from one place to another).
There may be many causes for this:
1. Your Anti Virus was unable to make an Exception in it's Firewall Settings for Minecraft. ( In that case you must do it manually )
2. Your ISP Speed is not at par with the distance between you and Server Location. ( You need to upgrade the Speed sadly )
3. There is an internal disturbance in the connection between your Computer and the Server. ( Out of your hands )
4. The Server is creating heavy traffic ( Many Players Joined ) and as a result your Internet Connection is getting dropped.
This issue sometimes get solved by itself ( Happened with me ) when all the above disturbances are resolved without your interference. Sadly, this issue can't be solved on your own in most cases.
I hope it helped out. Have a nice day. Let me know if there's anything else i can help you with.
