In its strategic modernization program, as well as in its opening arms control position, the Reagan administration could ultimately weaken the United States in conventional as well as strategic nuclear arms.
That is the verdict of Stansfield Turner, former director of Central Intelligence. The retired admiral says that instead of pushing for the heavy land-based MX missile, the US should opt for a new leg of the strategic triad - the cruise missile - in which the US has a clear 6- to 10-year head start over the Soviet Union.
''Cruise missiles and bombers are better than land-based ICBMs for a credible retaliatory strike,'' Mr. Turner told reporters over breakfast. They provide better deterrence, since they are more accurate and less vulnerable to a first strike than are ICBMs, he said.
And in conventional warfare, he said, the recent fighting in the South Atlantic and in Lebanon showed the importance of precision-guided munitions (including cruise missiles). Since the US has advanced faster and further than the Soviet Union in this area, he said, it could be forced into giving up its advantage in this area as a result of strategic arms reductions talks.
President Reagan's opening arms-reduction position does not cover cruise missiles directly, but leaves them on the negotiating table as a bargaining chip.
On other subjects, Mr. Turner made these points:
* ''Until the Reagan administration focuses its defense effort, then I am not in favor of the (defense spending) increases as planned,'' he said. Building two new nuclear aircraft carriers, he said, ''will strap the Navy for the next decade . . . they won't be able to buy much else.''
''One ought to broaden one's outlook on what is national security,'' he told reporters. ''We cannot take advantage of the Soviet disarray because of our own economic disarray. Bringing down the deficit is part of national security, not just economic well-being, and if it takes cuts (in defense) to achieve it, then I'm in favor of it.''
Long an advocate of smaller carriers and the Harrier aircraft that proved so effective in the Falkland Islands, the former four-star admiral said he would reduce the increased procurement of US F-14 and F-15 aircraft. Also, he would not proceed with the new nuclear carriers and cancel the MX missile and B-1B bomber (while hastening development and production of so-called ''stealth'' technology for aircraft and missiles).
* Regarding the recent testing of Soviet ballistic missile and antimissile systems, he said: ''The Soviets take much more seriously than we that you have to be prepared for any military contingency, including nuclear war. I don't necessarily draw the conclusion that they intend to have that kind of war.''
* The departure of Alexander M. Haig Jr. ''will slow down'' the arms control process, with the Soviet Union sensing a hardening of US policy and hardening its own in response. ''I hope (Mr. Haig's resignation) will lead to a less anti-Soviet excuse for our policy in Central America.''
Mr. Turner describes Mr. Haig as having done ''a fine job,'' and as ''the most balanced man in the administration with respect to foreign policy.'' At the same time, ''everybody considers George Schultz (nominated to be Haig's successor) so capable that it may not be a net loss to the country.''