The Defense Department has classified a report on President Reagan's Strategic Defense Initiative (SDI) that was compiled entirely from unclassified information, according to several senators.

The General Accounting Office, the investigative arm of Congress, completed the report on the structure and budget of the SDI Organization Jan. 10. More than a month later, the SDI office stamped the report secret, the senators said in letters to Defense Secretary Caspar W. Weinberger and Lt. Gen. James A. Abrahamson, head of the SDI office.

"We are concerned about your classification of this document and whether it signals a change in your policy on disclosure," Sens. J. Bennett Johnston (D-La.) and Lawton Chiles (D-Fla.) said in a letter to Abrahamson. Chiles and Sens. William Proxmire (D-Wis.) and Mark Andrews (R-N.D.) sent a similar inquiry to Weinberger.

An SDI spokesman, Lt. Col. Lee DeLorme, said the report was classified legitimately. He said pieces of unclassified data may have damaging consequences if compiled and released with government approval.

"The Russians could put them together, but then they would never be certain of what they have," he said. "By compiling it all and putting it all in a single government document, you have made their task much easier."