How to prevent root/ IK bones causing strange deformation of mesh?

Hello everyone, I hope that someone can help me about a tiny question. Many thanks in advance!:o

I am a beginner on modeling/Rigging/Skinning. I I have created a character and tried to bind it with the default mannequin’s skeleton.
I didn’t change the skeleton’s any joints. I just made a quick binding with the character mesh imported.
When I test the binded model, there’s always some strange deformation of mesh which seems to be caused by IK Root/joints,as shown in below picture.
What should I do to prevent this? I have already put the weight of all IK bones and root into 0.

I am sure it’s a extremly simple point to prevent this, but I have no idea where to find the correct part to learn about.
I would be grateful for any help.
Thank you in advance.