Celine Goberville of France looked on course to spring a surprise and dethrone the reigning Olympic champion as she led going into the final shot.
But her last effort was wayward, scoring a lowly 8.8, leaving Guo to nail a near-perfect 10.8 to keep her title.
Goberville did manage to hold her nerve in a shoot-off against Olena Kostevych to claim silver, France's first medal of the Games.
Kostevych had to settle for bronze, which also put Ukraine on the medals board for the first time.
Guo said: 'I'm very excited and very happy.
'There are 10 shots and every one counts. My first shot scored just eight. Every shot was very important.'
Goberville said: 'The last shot was a mistake but I didn't want to become obsessed by it.'