Port '80' is in use

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?