A federal judge here issued a key ruling yesterday that assures that a jury now being selected--and the public--can hear the broadest possible description from government psychiatrists of John W. Hinckley Jr.'s state of mind a year ago when he tried to assassinate President Reagan.

Hinckley's defense lawyers had argued that the psychiatrists' testimony should be excluded, or at least restricted, because it was tainted by their knowledge of statements Hinckley made after his arrest and of documents seized from his cell at a federal prison.

The courts have said that evidence was obtained illegally from Hinckley by FBI agents and prison officials.

At a rare weekend court session yesterday, however, the psychiatrists told U.S. District Judge Barrington D. Parker that their opinions on Hinckley's mental state were derived from a wealth of other information, including Hinckley's own voluminous personal writings and scores of interviews with Hinckley himself.

Afterward, Parker denied the defense request. "From what the doctors are saying," the judge said, "they could have very well arrived at their judgments, their conclusions and their analysis without the information that is the subject of this inquiry."

The prosecution has said that the psychiatrists who testified yesterday, including Jonas Rappeport of Baltimore, have concluded that Hinckley suffered from no serious mental illness on the day he shot Reagan.

Rappeport had examined Arthur Bremmer, who shot former Alabama governor George C. Wallace, and Sara Jane Moore, who pleaded guilty to the attempted assassination of former president Gerald R. Ford.

In another crucial ruling yesterday, Parker decided that for all 13 criminal charges, it will be up to the prosecution to prove beyond a reasonable doubt that Hinckley was sane on March 30, 1981 when he wounded Reagan and three others outside the Washington Hilton Hotel.

Hinckley, who was present at yesterday's four-hour hearing, contends that he was legally insane at the time of the shootings. To accept that defense, the jury must find that he suffered from a mental illness and as a result, could not abide by the law or could not appreciate that his actions were wrong. Jury selection for his trial, which began last Tuesday, will resume Monday in U.S. District Court here.

It has been well-established in the federal courts that once a defendant charged with a federal crime raises some evidence of insanity, the burden shifts to the prosecution to prove that the defendant was sane and thus criminally responsible for his acts.

The question of the burden of proof became an issue in Hinckley's case because he is charged not only with federal offenses relating to his assault on the president and Secret Service agent Timothy J. McCarthy, but also with local assault charges stemming from the shooting of White House press secretary James S. Brady and D.C. police officer Thomas K. Delahanty.

The prosecution, relying on a recent federal appellate court opinion, agreed that it had the burden of proof for the federal offenses in Hinckley's case. But it contended that on the local charges, the defense would have to prove that Hinckley was insane, by a preponderance of the evidence.

Hinckley's lawyers countered that such a division of proof would require two different instructions to the jury and would create "obvious confusion" when they tried to come up with a verdict, thus denying Hinckley his constitutional right to a fair trial.

Moreover, the defense argued that when the government decides to mingle the federal and local charges in an indictment--as they did against Hinckley--the federal law on the burden of proof prevails.

Parker said he could find nothing in the appeals court decision to compel him to make the defense carry part of the burden of proof at Hinckley's trial.

At yesterday's hearing, Parker also said that he will reject any attempt by Hinckley's defense lawyers to argue that Hinckley's state of mind was so diminished at the time of the shootings that he did not have the mental capacity to intend to commit the crimes. This argument, just short of insanity, is called "diminished capacity."

In court papers, the government had contended that Hinckley's lawyers were making an "unwarranted effort to present two insanity defenses" by trying to supplement their insanity defense by giving the jury a chance to consider a second theory of diminished capacity.

The government had argued that the courts have allowed the introduction of that concept only in first-degree murder cases where the intent question focuses on proof of premeditation and deliberation--which are not relevant in Hinckley's case.

Parker, who issued all of yesterday's rulings in open court, said that Hinckley's lawyers "had cited no case in this judicial circuit in which diminished capacity" was applied to a crime other than first-degree murder.