Hi @Guan_Haibin,
Thank you for reporting. This not a user error, it looks like the EMP format is not properly equipped for Phred64 format, and so is being interpreted as Phred33. I have raised an issue to get this fixed.
You could try converting to Phred33, or else @thermokarst is going to follow up with a different workaround.
Also, just a suggestion: mock-6 is a really old and not particularly good dataset in mockrobiota. If you want 16S data, I would recommend just about any others. mock-12 and mock-18+ are particularly good (and probably phred33 already).
Good luck!