White House officials today played down the importance of a near-collision in the airspace above President Reagan's ranch, but the question of presidential security at Reagan's mountaintop retreat has long been troublesome for the Secret Service.

This worry was not reflected in the daily briefing here. White House spokesman Marlin Fitzwater, asked whether new security procedures would be adopted, joked, "You mean like a 6,000-foot steel wire fence, or what do you have in mind?"

On Wednesday, a light plane that violated the restricted airspace above the ranch 20 aerial miles northwest of here flew within 150 feet of the helicopter carrying Reagan. The helicopter, Marine One, was descending to land at the ranch.

The ranch is called Rancho del Cielo, or Heavenly Ranch, but protecting it has been anything but a heavenly task for the Secret Service. White House officials are reluctant to discuss specific security procedures, but a high-ranking administration source said early in the Reagan presidency that the mountaintop retreat posed a "security nightmare."

While the ranch is remote and accessible by car only on a narrow, winding road, it is exposed to the light planes that fly frequently along the California coast. Reagan often works outside or rides while he is at the ranch, where he had spent 268 days of his presidency before this vacation.

Fitzwater was asked today if he thinks that security for the president at the ranch is adequate and said, "Yes, absolutely."

But when reporters asked whether additional measures are needed to protect against an inadvertent aerial intruder or a potential terrorist, he ridiculed their premises.

"How do you know the intentions of every pilot in the sky?" Fitzwater asked. "There's thousands of planes . . . . Do you want to close down aviation in America? Do you want to interview every pilot before he gets in his plane. What do you want to do? Do you want to put Stingers {surface-to-air missiles} in the tail of Air Force One? What do you want?"

When the White House spokeman was then asked what would stop another plane from intruding into the restricted airspace, he replied, "Nothing."

Secret Service and aviation officials said pilots should know enough to keep out of the zone surrounding the president's ranch, which is marked clearly on aeronautical charts.

"It's safe to assume that a good experienced pilot . . . would be aware of restricted airspace and would not enter it," Secret Service spokesman Richard Adams said.

"There's no reason that a pilot should blunder into the area because it's well advertised," said Gerald Walton, an official with the Federal Aviation Administration's air traffic division in southern California.

But, he added, "If a pilot wants to intentionally violate the airspace, we would have no way of preventing the pilot from doing that. The only thing we can do is take whatever enforcement action is available to us after the fact."

Walton said the FAA, which continuously monitors the nation's airspace, has no procedures in effect to alert military forces if an intruder penetrates the prohibited zone above the president's ranch.

Given the heavy air traffic in the area, he added, it is impossible to tell whether a violation of the airspace is imminent. "By the time we are aware that it is going to happen, it has happened," he said.

White House officials are hopeful that the FAA's prompt action in revoking the pilot's license of Ralph W. Myers, who flew the plane that nearly collided with the presidential helicopter, will act as a deterrent.

White House chief of staff Howard H. Baker Jr. said today that Reagan was not aware of the incident when it occurred and did "not seem particularly upset about it" when Baker telephoned him to tell him about it. None of the passengers in the helicopter, which included national security adviser Frank C. Carlucci as well as the president, Baker and Fitzwater, was aware of the incident until afterward.

White House sources said, however, that Nancy Reagan, who arrived separately at the ranch after visiting her mother in Phoenix, heard the report of the near-collision on the radio. "She was naturally quite concerned about it," one of the sources said.

Staff writer David S. Hilzenrath contributed to this report from Washington.