Yu Hai's deflected free-kick 10 minutes from time proved to be the winner as the ball bounced off Nawaf Alabid, with the change in direction leaving scrambling Saudi goalkeeper Waleed Abdullah in no-man's-land.
The match was tedious in the first half but sprung to life after the interval as Saudi Arabia should have taken the lead with a 60th minute penalty.
Iranian referee Alireza Faghani pointed straight to the spot when China's Ren Hang brought down Naif Hazazi, who surged into the box after a run down the left.
But when Hazizi stepped up to convert his own spot kick, his meagre effort was saved easily by China goalkeeper Wang Da Lei, who was celebrating his 26th birthday
Saudi Arabia's only other chances were speculative, underlining how significant the absence of controversial striker Nasser Al-Shamrani will be to its Asian Cup chances.
Al-Shamrani, the Asian Player of the Year, was withdrawn from the tournament on Friday evening with an abdominal injury, and the Green Falcons lacked spark without him.
There were few genuine chances in a testy first half in which the Chinese grew in both confidence and fluency.
Just past the half-hour mark, right winger Zhang Chengdong beat two Saudi defenders and cut back for Wu Xi, but his shot was blocked by Osama Hawsawi.
Wu is usually a right-back or defensive midfielder for his club side Jiangsu Sainty but was used in a more advanced role in Brisbane - one of several players used out of position by coach Alain Perrin.
He had another golden opportunity to put China in front 10 minutes later but his attempt from the top of the box was parried wide by Saudi gloveman Waleed Abdullah, although a corner kick was not given.
The three points took China, which failed to get out of the group stage in 2007 and 2001, joint top of Group B alongside Uzbekistan.
