I searched the google and youtube all day.
Now, it works.
Softether VPN Server IP: x.x.x.x
Mikrotik’s WAN IP: y.y.y.y
Create Ipsec proposal first:
IP->IPSec->Proposals
Then create ipsec peer:
IP->IPSec->Peers
And create ipsec policy:
IP->IPSec->Policies
Finally create L2TP Client:
Interfaces->Interface->Add L2TP Client
Other configuration options for Firewall and Routes:
