VANDENBERG AIR FORCE BASE, Calif. (AP) _ An unarmed MX missile was tested over the Pacific Ocean Thursday, but an experimental radar decoy device it carried failed to operate, the Air Force announced.

The missile blasted off from Vandenberg Air Force Base at 5:30 a.m. and its six dummy warheads splashed into the sea in the Pacific Missile Test range about 30 minutes later, said Maj. Barry Glickman. The dummy warheads landed in the Pacific near Kwajalein Atoll, about 4,100 miles away from Vandenberg.

In time of war, the so-called re-entry vehicles would carry nuclear warheads.

''It went right where it was supposed to go. It was a very successful launch,'' said Glickman, who is with the ballistic program at Norton Air Force Base in San Bernardino County, 60 miles east of Los Angeles.

However, Glickman said, a platform carrying decoy devices intended to confuse enemy radar failed to work. The decoys aren't part of the MX system, but were being tested as part of a program to develop improvements for all American ballistic missiles, including the Minuteman, Glickman said.

The MX, dubbed the ''Peacekeeper'' by President Reagan, is the nation's latest intercontinental ballistic missile, designed to deliver several independently targeted bombs to locations hundreds of miles apart.

The Air Force expects to have 10 of the missiles installed in underground silos at Francis E. Warren Air Force Base in Wyoming by the end of the year, Glickman said. Congress has authorized deployment of 50 of the missiles.

Thursday's launch was the 14th in a series of 20 test flights, Glickman said.

Vandenberg is 140 miles northwest of downtown Los Angeles.