The American nuclear submarine that collided with a Japanese freighter last month failed to aid the striken Japanese ship because the submarine commander did not realize the freighter was sinking, the Navy said yesterday in a preliminary report on the incident.

This same failure to realize that the freightor had sunk also was largely responsible for "excessive delays" by U.S. naval officials in reporting the collision to the Japanese government, the report said.

The report, made public yesterday by the Navy, is an attempt to answer some of the questions surrounding the April 9 collision between the nuclear submarine USS George Washington and the freighter Nissho Maru in the East China Sea off the southwest coast of Japan.

The captain and another crewman of the freightor were killed in the accident. Thirteen other crewmen were rescued by a Japanese destroyer 18 hours after the collision.

The incident caused an uproar in Japan and led to a mesage expressing his personal regrets from President Reagan to Japanese Prime Minister Zenko Suzuki. Suzuki, who arrived in New York yesterday and was to be given a copy of the preliminary report, is to see Reagan Thursday. State Department officials said the report should go a long way toward soothing Japanese concerns over the incident.

According to the report, the George Washington was attempting to evade detection by a search plane as part of a training exercise on the morning of the collision.

The ship's sonar equipment, which was functioning properly, did not detect the freighter until right before the collision. The sonar information was relayed to the submarine's deck officer, but it was acknowledged or acted upon.

However, the report added, "It is not clear that the collision could have been avoided had the sonar information been acted upon promptly."

After the collision, the submarine came to the surface for three to five minutes, during which time its captain "did not note any signs that the [Nissho Mari] was in distress," the report said.

"He was no signs of panic or disruption," the report continued. "Based on his brief observations of Nissho Maru and his evaluation of the physical jolt experienced by the George Washington, the commanding officer concluded that the Nissho Maru had not been seriously damaged."

The Navy has said it accept full liability for the collision and expects to settle all claims satisfactorily.

The George Washington sailed from Guam yesterday under a new commanding officer. The submarine's captain at the time of the collision, Cdr. Robert D. Woehl, and its watch officer, Lt. R. D. Hampton, have been "temporiarly reassigned" during the investigation. Navy officials stressed that these transfers did not imply culpability.