I have hear that batching draw calls is a commonly used technique to optimize mobile games,like unity,but as far as I know,for one draw call,the draw buffer must be continuous,so does it mean we have to move draw data around to make them lay continuously in one buffer,that will be a great overhead,am I right

‘Batching draw calls’ could just mean to sort objects by materials, so you still have draw call per object, but you change shaders less frequently.
The next step is to use a texture atlas or texture arrays, so you change textures less often as well.
Only after that merging geometry as you say becomes next, which is no overhead if you do it as an offline pre-process, e.g. for static geometry.

You made it very clear,thanks a lot.