Please see attachment, this is one circulation electric heater, I want to calculate the pressure drop when the fluid flow through this heater, I setup 3 level and another 4 level in initial mesh, but produce difference pressure drop? More level in initial mesh, more pressure drop happened? Could you tell me the reasons?

You need to take a close look at your mesh. Especially with a model like this, where you have a high level of fine detail. Make sure your mesh accurately captures the geometry around the pipes. Use mesh cut plots and be sure to turn off the "Use CAD geometry" in the plot options so you can see exactly what your mesh looks like.

To be honest I'm not sure you will be able to do a complete analysis of this geometry, the required mesh cell count must be in the dozens of millions. I would try to simplify the model somehow, maybe a porous medium in place of the tubes.