The 21-year-old ACT rider dominated the time trial event at the worlds in Birmingham, England and was the fastest for the two runs.
Great British rival Shanaze Reade finished second.
Buchanan is now favourite for the match racing, to be held on Saturday.
Buchanan is the world No.1 and earlier this year became the first Australian woman to win a Supercross round.
She is concentrating on BMX for the Olympics after winning the 2009 and '10 world titles in four cross, a mountain bike discipline.
Buchanan was the only Australian medallist for the elite and junior time trial events.
World No.2 Sam Willoughby was sixth in the men's elite event, won by American Connor Fields.
Of the other Olympic hopefuls, Lauren Reynolds was 14th and Melinda McLeod was 18th, while Brian Kirkham finished eighth and Khalen Young took 26th.
Bodi Turner was the top Australian in the junior men with sixth and Rachel Jones was fifth in the junior women time trials.