Why password authentication failed for user "postgres"?

[BeginnerC <chuxuec@xxxxxxxxxxx>]

Hello everyone,

I am a newbie to the postgres,when I use the psql to connect to the postgres,a error message printed:

These command list like this:

psql -U postgres

Password for user postgres:postgres

postgreSQL: password authentication failed for user "postgres"

How to solve this problem?

Thanks in advance!