The 23-year-old swimmer, clocked four minutes 16.54 seconds to claim the silver medal at the 26th SEA Games swimming event held at Jakabaring Aquatic Centre, here, tonight.
The qualifying mark for the London Olympic Games is 4:18.75s.
"I am happy with the result as this is what I've wanted. I've been trying to qualify for the Olympics in the event," she said.
Thus far, she had qualified for the 800m freestyle when she surpassed the 8:51.82s qualifying mark under Category B at the national championship held at the National Aquatic Centre in Bukit Jalil with the time of 8:49.28s.
At the Beijing Olympics in 2008, Cai Lin took part in the 400m and 800m freestyle.
On the silver medal she won tonight, Cai Lin said she was happy that her SEA Games record of 4:10.75s achieved at the Laos SEA Games in 2009 was still intact with her.
Besides Cai Lin, the Malaysian swimming squad clinched another silver medal through Chan Kah Yan in the women's 50m backstroke.
She managed to erase her own national record of 29.70s with a new record of 29.55s.
The gold medal was won by Shana Lim of Singapore in 29.37s, while Dorothy Grace Hong of the Philippines took the bronze with the time of 29.96s.
Meanwhile, the men's 4x100m freestyle quartet -- Foo Jian Beng, Kevin Yeap, Vernon Lee and Lim Ching Hwang -- was disqualified due to false start.