Olympic champion Shiffrin has already won the slalom crystal globe and Sunday was her record-extending 109th World Cup victory across all disciplines.
The American's closest rival Emma Aicher of Germany was second in the race in Are, Sweden, ahead of Switzerland's Wendy Holdener.
There are four races remaining, worth a total of 400 points, at next week's World Cup finals in Norway.
Shiffrin, chasing a sixth overall Crystal Globe, led Aicher by 0.51 of a second in Sunday's first run and won by a combined 0.94.
The American now has 1,286 points to Aicher's 1,146 with Switzerland's Camille Rast on 989.