I solved the problem myself by the following steps:
1) wipe out the handheld data(Options -> Security Options -> General Settings -> Menu -> Wipe Handheld)
2) Delete the use in BES manager
3) Stop BES services ( Controller, Router, Dispatcher, Synchronization, Policy)
4) Run outlook as my Besadmin account, tools -> Send/Receive -> Send/Receive All
5) Run outlook as the user account, tools -> Send/Receive -> Send/Receive All
6) Restart the BES services
7) Readd the user in BES manager
8) Reset the user's activation password
9) On handheld, Options -> Advanced Options -> Enterprise Activation to reactivate the user |