Victory in the eight-team tournament itself will guarantee a place in the ICC Trophy in Ireland in July which is the final qualifier for the 2007 World Cup in the Caribbean.
Having won the toss and elected to field, Qatar restricted Cayman Islands to 207 runs at the Selangor Turf Club as Omer Taj took 4 for 35.
In reply, Qatar captain Azhar Iqbal guided his side to victory, top scoring with 61 off 83 balls.
After the match, Iqbal expressed his delight that his side had qualified for the semi-finals with one match to spare.
"It was a tough match, the Cayman Islands have a very strong team and the hot and humid conditions are a challenge," said Iqbal.
"We are used to the heat in Qatar but not the humidity. We won the toss and bowled well on a good batting wicket before playing very well to chase down the total. We are confident looking ahead and feel we have a good chance in the rest of our matches."
Qatar's final Group B match is against Fiji who also qualified for the semi-finals on Tuesday with a 112-run win over Zambia.
The experienced Fijian all-rounder Neil Maxwell was Man of the Match for the second game in succession.
Group A is wide open with all four teams still capable of progressing to the semi-finals.
Papua New Guinea (PNG) are in pole position after a five wicket victory over Italy left it as the only unbeaten team in the group after two rounds of matches.
PNG must still win their remaining match against Kuwait to be certain of making the last four.
Nepal beat Kuwait by five wickets on Tuesday to claim their first points of the tournament and must win again against Italy on Thursday to keep their hopes of qualifying for the ICC Trophy alive.