The Shuberts will take over management of the 146-year-old National Theatre this spring under an agreement signed yesterday with the non-profit New National Theatre Corp.

With the agreement comes the promise of a half-million-dollar refurbishing of the 1,600-seat theater, top-drawer bookings and a bright future on the new Pennsylvania Avenue.

The five-year agreement will become final with court approval of a modification-of-consent decree. The Shubert Organization, which now operates in five cities, is required under a 1958 ruling to receive court clearance before expanding into a new area.

The volunteer board will continue to operate the National but will enlist the Shuberts, with their expertise and contacts in the theatrical world, for booking productions.

Earlier this month, the board ended a five-year booking relationship with the Kennedy Center. The National has been dark six months in the last year.

At a press conference yesterday, Gerald Schoenfeld, board chairman of the Shubert Organization, said the renovation is scheduled to be complete by the end of the summer, with the Shuberts spending "upwards to $500,000 and maybe more."

"We intend to bring the best of theater here . . . We're not going to fill time with garbage," Bernard Jacobs, the Shubert president, emphasized yesterday.

Jacobs mentioned "Amadeus," the much-talked-about current London hit, as a strong possibility for the opening-night attraction at the National this fall under new management. The play about Mozart is by Peter Shaffer, who also wrote "Equus" and "The Royal Hunt of the Sun."

"We hope that will be our first show. If it is produced in America next season, it will come here first," he promised.

For the time until the court decision on the modification-of-consent decree, the board has booked two shows into the National.

"The Kingfisher," with Rex Harrison and Claudette Colbert, will open next month for a four-week stand to be followed by Bob Fosse's "Dancin'" for five weeks.

Harry Teter, an attorney who has become a volunteer impresario, said yesterday that other productions tentatively planned are "Evita" and "They're Playing Our Song."

The Shuberts, as managers, will split proceeds with the nonprofit New National Theater Corp.

Yesterday Maurice Tobin, president of the corporation, talked about plans for a new multiproject National, with lectures, seminars and televised openings and programs.