After continuous investigation and research, I found that it was the problem of powernap. It's not bigsur's bug that cause external devices wake up automatically. Unlike other Intel MacBook,powernap can't be completely disabled on M1.
Typing the command in terminal.app "pmset -g log | grep darkwake" to view the darkwake log, I found that two events caused my m1 MacBook Pro to wake up. As follows:
1. DarkWake from Deep Idle [CDNP] : due to SMC.OutboxNotEmpty smc.70070000 wifibt wlan/ Using AC (Charge:100%) 6 secs
2.DarkWake from Deep Idle [CDNPB] : due to NUB.SPMISw3IRQ nub-spmi.0x02 rtc/Maintenance Using AC (Charge:92%) 45 secs
Both events are caused by M1 MacBook forcing powernap to be enabled. But we also have a solution (need to upgrade to the latest macOS 11.2 RC):
1.Run command in terminal: sudo pmset -a tcpkeepalive 0
2.Run command in terminal: sudo pmset -a powernap 0
It's not over yet, because another process will still wake up in darkwake mode. To shut down this process, first you need to disable SIP: https://developer.apple.com/documentation/security/disabling_and_enabling_system_integrity_protection
Then run command in terminal: sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.PowerUIAgent.plist
BTW,do not re-enable SIP or last command will be invalid.The recovery mode of M1 is different from Intel, please search for it yourself.