thanks!
I also talked to the Cisco technician, she point out that the traffice from Site A to Site C is dropped by ASA firewall at Site B because the VPN phase 2 ESP value issue. And she asked me to check the configuration on Juniper.
She told me that she can see the "esp" value from Site A to Site B (ASA), but there is no esp value passing through from Site A to Site C.