"I can't believe I swum that fast," she said after clocking 24.13 seconds in the London Aquatics Centre pool. "I was in shock, total shock. I'm gobsmacked."
Halsall had already qualified for the London Games in the 100 butterfly and 100 freestyle and, with the sprint relays, could be chasing five medals at her home Games starting on July 27.
Her win on Saturday, in a distance that is not her strongest, made her the only British female swimmer to qualify for three individual events.
The world record time in the 50 freestyle is held by Germany's Britta Steffen, 23.73 set at the 2009 Rome world championships in a now-banned type of hi-tech swimsuit.
Sweden's Therese Alshammar won the world title in 24.14 in Shanghai last July.
"I will be happy to just get one (gold)," said Halsall, the European 100 freestyle champion who was 50 runner-up in the 2009 world championships in 24.11 and has now almost matched that hi-tech time with one set in a regular textile suit.
The 21-year-old, back in black after wearing a bright pink swimsuit in Thursday's 100 freestyle so her father could spot her, said she was in the best shape she had ever been at this stage of the year.
"I'm just full of confidence now and looking forward to having a little bit of a break," said Halsall.
"If I can keep this momentum going into the summer I can achieve great things."
Halsall said the crowd, even if their numbers at the test event were a fraction of the 17,000 expected at Games time, had given her a huge boost in a race that is all about split seconds.
"The more you get lifted by the crowd, those fractions turn in your favour," she said.
Thirty eight British swimmers qualified for the Games at the trials, their numbers divided equally between men and women, but the total could increase to 47 with some berths yet to be filled in the pool and open water.
Of the selected group, 20 hold British records and 12 have won world championship medals. They include double Olympic champion Rebecca Adlington who qualified for the 400 and 800 freestyle.
British swimmers won six medals at the Beijing Games four years ago and the target is to match that at least.