The Pershing II battlefield missile will have a reliability rate of between "80 and 90 percent" by the time it is deployed in West Germany this December, Undersecretary of the Army James R. Ambrose said yesterday.

The Army executive acknowledged that the Pershing II had failed a number of times in its flight tests, but said that its record was "neither the best nor the worst" for missiles of similar size.

"We will not deploy this kind of weapon," Ambrose said of Pershing, unless its probability of success when fired under combat conditions is at least 80 percent.

The Pershing is scheduled to be deployed in West Germany, where it will pose a nuclear threat to eastern European nations, including the Soviet Union. Ambrose said that the terminal guidance system in the Pershing makes it a highly accurate weapon that could reach its targets in a war within 10 minutes.

The Soviet Union has protested the planned deployment of the Pershing, saying that its short flight time makes it a first-strike weapon suitable for surprise attack. President Reagan has rejected that argument, saying that it is the Soviets, with their SS20 and other medium-range missiles, who pose the bigger threat to peace in Europe.

Ambrose, in a discussion with Pentagon reporters, also discussed another Army missile that is having problems: the Patriot antiaircraft rocket slated to be deployed in western Europe as well. He said the radar that would guide that missile to enemy planes flying at high altitude has failed time after time.

As a result of the Patriot's frequent failures during testing, the undersecretary said, the Army has revamped the schedule for the missile. He said the contractors will have to achieve a series of "milestones" in perfecting the Patriot.

This segmented schedule, he added, will delay the planned deployment of Patriot by about five months. The new target date for the antiaircraft missile being deployed in Europe is September, 1984.

Ambrose called the Patriot "quite essential" to the defense of Europe.