The Supreme Court yesterday left untouched a ruling upholding the constitutionality of the controversial D.C. preventive detention law.

The action, taken without comment, means the law can continue in effect and, more importantly, leaves the Reagan administration and state governments free to experiment with their own preventive detention proposals.

It leaves unanswered the difficult constitutional questions raised by the proposals.

The court's refusal to review the May 1981 ruling of the D.C. Court of Appeals has been expected since earlier this month, when the justices disposed of a Nebraska right-to-bail case without ruling on the merits. The court said the Nebraska case was moot, because the defendant already had been convicted and pretrial release no longer was an issue. The D.C. defendant was in the same circumstance.

The action also appeared to be a sign that the court simply did not want to cope with the controversial subject this term.

The D.C. preventive detention law allows some accused persons to be held in jail without bond on the grounds that their past records and their potential for committing more crimes make them too dangerous for pretrial release.

The constitutionality of preventive detention is one of the most vigorously debated of all law-and-order issues because of the Eighth Amendment's prohibiton against unreasonable bail, and because its critics say it breaches the principle that a defendant is presumed to be innocent until he is proved guilty.

In fact, many civil liberties lawyers are pleased that the current Supreme Court is not reviewing preventive detention. Because of what they perceive to be the court's conservative slant, they fear a ruling that would give the states a cue to enact even more extreme detention proposals.