How can I fix the this problem . I can’t put the helmet over the head of my character 1.12.2

did you animate the model? Animate it with head movement animation, so it will tilt where you're looking. That fixes it slipping off a moving head. Now just reposition the model until you get it to be on your head.

If you are using blockbench a simple way to do it is select "Skin" type, then fashion your model around the head of the Steve (select steve for the model skin) then convert to modded entity and delete the entire except for the helmet, then export it as java or obj or whatever kind you are using.

1.12.2.

no support

change your model code to this:

Headphone.setRotationPoint(0.0F, 0.0F, 0.0F);
Headphone.cubeList.add(new ModelBox(Headphone, 0, 0, 4.0F, -9.0F, -0.5F, 1, 4, 1, 0.0F, false));
Headphone.cubeList.add(new ModelBox(Headphone, 0, 0, -5.0F, -9.0F, -0.5F, 1, 4, 1, 0.0F, false));
Headphone.cubeList.add(new ModelBox(Headphone, 0, 0, -6.0F, -5.0F, -1.5F, 2, 3, 3, 0.0F, false));
Headphone.cubeList.add(new ModelBox(Headphone, 0, 0, 4.0F, -5.0F, -1.5F, 2, 3, 3, 0.0F, false));
Headphone.cubeList.add(new ModelBox(Headphone, 0, 0, -5.0F, -9.0F, -0.5F, 10, 1, 1, 0.0F, false));

i just loaded your model into blockbench, changed the pivot point of the whole helmet to the bottom center of the head, and exported the model