Visa appointments were cancelled and members of the public queing to enter were ordered to leave as the consulate scrambled to react to the incident, AFP reported.
Uniformed police officers were also seen donning gas masks outside the building in the busy Central district of the southern Chinese financial hub.
Four police vans and a bomb disposal truck were seen deployed at the sealed-off main entrance.
However, the consulate later said the powder was not found to be hazardous.
'We can confirm that an envelope containing white powder was opened inside the Consulate General today,' a spokesperson for the US consulate said.
'The incident has now been resolved. Visa appointments for the rest of the day remain cancelled,' the spokesperson added.
'As always, our highest priority is the safety of our personnel and visitors.'