as the gif shows
Hi @banguxizhi85 and welcome to the forums! 
Everything looks correct; the symmetry button is active and the shapes appear to be symmetrical. Can you share what steps you took to get to here? Is it possible the shapes are symmetrical across a different axis, such as Z or Y? You can change the axis at the top of the viewport, where it says “Pose X”.
Is it possible that the character was in a different pose when you ran the “Assign and Connect” command? Symmetry is based on the position they are in when you first assign.
If you follow these steps, does symmetry also not work?
I compared my own file with your tutorial, and found that your number is 1, but it is 0 here. I don’t know how to adjust it to 1. I tried to drag the mouse and there was no response.
I set up the rag doll again and found the reason, because I didn’t choose the controller of the neck when I made the arm. happy and thanks
Oh yes, symmetry is limited to Markers in the same group.
Edit: The reason is because there can be 2 or more characters in your scene, and you would normally only expect to manipulate the other side of the same character, despite their symmetrical positions being identical.