Port '80' is in use

Developer help Self-hosting
1

First of all, Nothing runs on my port 80.
After change the server port in the config.development.json to 80, and run ‘ghost start’, It shows the error:

1) Validating config

Error detected in the development configuration.

Message: Port '80' is in use.
Configuration Key(s): server.port
Current Value(s): 80

I tested a python server on port 80, nothing went wrong
So for ghost, if I want to change the port to 80, what should I do?