The 32-year-old opened his World Cup season with victory in the parallel giant slalom in Carezza last month and took his second win in a row as the circuit moved to Austria.
Prommegger, who won the parallel slalom World Cup title last season, reached the big final with relative ease before seeing off the charge of Italian Fischnaller by 0.11seconds.
Meanwhile Zan Kosir, who lost to Prommegger by 0.61 in the semi-final, comfortably beat American Justin Reiter by 2.43 in the small final to take third position in Bad Gastein.
Meanwhile in the women's parallel slalom event, last year's World Cup runner-cup Amelie Kober won the second successive race she has started on the circuit.
Kober, victorious in the parallel giant slalom at World Cup finale last season, was just too good for home hope Marion Kreiner in the big final as she won by 0.33.
And, while Kreiner couldn't beat Kober, her Austrian teammate Claudia Riegler was awarded victory against Natalia Soboleva in the small final for third after the Russian was disqualified.