The mobile phone automatically turns off and then on after sending a short message. What happened?

Which one is so poisonous! There are two possible reasons. One: the battery of the mobile phone is loose, so you can drop it gently on the mobile phone. If it is turned off, the battery is loose. You can just put a piece of paper on it, or take off the back cover to see if the battery is loose. Two: the connection is difficult, and you can brush the machine for free after the warranty period. The first possibility is very high.