A month after President Reagan's April announcement that he would be content with only 100 MX missiles, Air Force planners were writing that this was only the first phase in likely MX deployment, according to a secret document obtained by The Washington Post.

The document, called a Program Objectives Memorandum, or POM, is designed to guide Air Force budgeting and planning for the five fiscal years 1985 through 1989.

In an effort to rescue the MX after a disenchanted Congress sidetracked it last winter, Reagan on April 19 announced a new weapons program. He would build only 100 of the 10-warhead MXs and turn in the future toward a smaller, less threatening mobile missile that might be more conducive to arms control.

Congressional moderates hailed this as a sign of restraint and funds for the MX were restored.

But the POM, dated May 11, speaks of the 100-MX deployment as only a first stage, and describes a "phase 3 follow-on deployment of the MX."

The Air Force said yesterday that this POM has been "superseded" by one that conforms fully to the president's 100-MX proposal.

But several lawmakers know about the POM and are expected to make an issue of it in opposing the MX when further defense bills come up for debate after the July 4 recess. They say the POM shows the Air Force still has a secret agenda that calls for deployment of at least 200 of the missiles, the same total sought during the Carter administration.

The POM says: "The need for improved ICBM intercontinental ballistic missile capabilities in the mid-1980s and beyond is addressed in the POM as a three-phased approach to ICBM modernization.

"Phase 1 is the near term deployment of 100 Peacekeepers," Reagan's name for the MX, "in Minuteman silos. This phase is essential to redress the existing imbalance in prompt hard target capability by adding 700 warheads."

The 700 are the 1,000 MX warheads minus 300 Minuteman warheads to be lost as 100 Minuteman missiles are taken out of the silos to make room for the MX. "It will demonstrate our resolve and help to ensure meaningful arms control negotiations," the POM says.

"Phase 2 continues R&D research and development on options to enhance survivability. These options include superhard silos, deep basing, small missiles and hardened mobile launchers.

"Phase 3 involves a follow-on deployment of additional Peackeepers, plus research and development on a new, small, follow-on ICBM . . . ." Other parts of the POM indicate that the third phase would involve deploying a second 100 MX missiles.

The Air Force provided this response yesterday when queried about the additional MX missiles described in the May 11 POM:

"The Air Force Program Objectives Memorandum is a classified document, and we are unable to discuss specific elements of it. However, the Air Force fully supports the report of the President's Commission on Strategic Forces which the president endorsed and sent to the Congress.

"This is reflected in the current version of the Air Force Extended Planning Annex, a section of the POM, and was forwarded to the secretary of defense on 22 June. It included the force level of Peacekeeper missiles recommended by the President's Commission on Strategic Forces. Any earlier drafts of the document have since been superseded."

One Air Force official who declined to be identified said the second hundred MXs got into the POM because somebody did not get the word in time about the president's strategic program.

A Pentagon civilian official, who also did not wish to be identified, said he doubted that the policy statements about a follow-on MX deployment got into the Air Force POM inadvertently. He contended that such general policy directions, in contrast to details that are revised as the POMs go through the budget process, are usually written carefully.