The present invention discloses a method for starting a single-stage system for nitrogen removal coupling partial denitrification and anammox. The two nitrogen removal processes are coupled by gradually enriching denitrifying bacteria in anammox granular sludge and then activating the partial denitrification process of the functional bacteria (denitrifying bacteria). Particularly, the method comprises: inoculating anammox granular sludge in a single-stage reactor, adding an organic carbon source in gradients to enrich denitrifying bacteria and to adapt anammox bacteria, and replacing nitrites in feed with nitrates gradually to activate the partial denitrification process.