Mayor Bloomberg will unveil a $47-billion-plus preliminary budget for fiscal 2006 today that will show a deficit that is only half the size he was predicting this time last year.

Aides familiar with the mayor’s budget told The New York Sun yesterday that Mr. Bloomberg will unveil a package that shows a $1.5 billion deficit, instead of the $3 billion deficit his budget experts had forecast for fiscal 2006 last January.

The city had underestimated the amount of revenues that would come in, one aide, who declined to be further identified, said. The boom in housing, a better-than-expected year on Wall Street, and a burgeoning local economy helped the city fill the gap.

In spite of the good news on the deficit, the mayor is expected to caution New Yorkers about the runaway fixed costs the city faces, such as Medicare and pensions, which cost the city nearly a $1 billion more than expected last year.

Budget analysts had predicted Mr. Bloomberg would have to cut city services drastically in fiscal 2006 because the deficit was expected to top $3 billion. By law, the city has to finish the year with a balanced budget. If it doesn’t come within $100 million of balance, a financial control board can take the reins.

The preliminary budget to be unveiled today is only the first step in a complicated process involving the mayor and the City Council. Mr. Bloomberg will offer a revised executive budget in April, the two sides will spend the spring horse-trading, and then the council will vote on the budget in June.