Welcome back @haoguang_shen
Couple of clarifying questions I have… You have character game object and you want to switch the model shown? If this the same project you were working on last week? (For anyone else’s reference)
I remember you were switching between 2 models with different materials and I suggested creating model each time. Are you disposing the original game object when you create the new one?
mesh.dispose(); If reloading is also too expensive of an option are you doing anything to cull the models/objects that are not being displayed on screen. Setting to inactive for example. This should help on the cost.
Let me know if this helps and feel free to ask follow up questions.
I will also throw out the old handy how to optimize your scene document. Really good resource just to have in general. Optimizing Your Scene | Babylon.js Documentation
Take a look at the “Scene with large number of meshes” and “Changing Mesh Culling Strategy” sections 
Let’s get this working at the fastest speed possible together!