Dear trongthinh,
If you plan to use DM-Crypt to encrypt your RootFS, then,The document "Root filesystem encryption using DM-Crypt" provides detailed steps to make a it.
The target was ARM64 you can adapt it to your need (Install ARM32 toolchain and compile for ARM target).
If this reply answers your question please mark it as "Correct Answer" so that other users can access it quickly.
Regards
Maro