Dear Xu,
If your interaction is significant, it means that there is a difference somewhere. The F-contrast will not tell you which it is, so to interpret the interaction run additional t-tests for the contrasts below: 
A1:B1 versus A1:B2
A2:B1 versus A2:B2
A3:B1 versus A3:B2
A1:B1 versus A2:B1
A2:B1 versus A3:B1
A1:B1 versus A3:B1
A1:B2 versus A2:B2
A2:B2 versus A3:B2
A1:B2 versus A3:B2

Date: Wed, 9 Sep 2015 10:45:38 +0200
From: [log in to unmask]
Subject: [SPM] 2*3 full factorial design
To: [log in to unmask]

Dear SPM experts,

My experiment is a 2*3 within-subject design. In the 2nd-level analysis, I use the 2*3 full factorial model. I found the brain activation in the interaction(F-contrast). However, I am not sure how to explain this 2*3 interaction?  Would you please give me more information about this? Thanks!

Best regards,