The Smithsonian yesterday mornign unveiled its new Discovery Theater with an appropriate first installment of its "Discover Puppets" series.

The new room in the Arts and Industries Building, 900 Jefferson Dr. SW, on the Mall, is a charming, versatile space in the area vacated for improvements many months ago.

Spacious, carpeted steps will be seating most of the 200-person audience, a relaxing place for children to Ioll, stretch or hunch as whims dictate. Chairs in the rear, also stepped, probably will be more comfortable for their elders. Masked yesterday by colorful curtains, the performing area can be expanded to complete the room's informal, circular effect.

To begin this room's promising life, the Smithsonian's Division of Performing Arts has created a novel exploration of puppetry. With Susan Linn of TV's "Mr. Rogers' Neighborhood," this first segment, to run through Jan. 7, is titled "Discover the Voice!"

Later will come episodes titled "Discover the Shap!," "Discover the Magic!," "Discover the Laughter!," and "Discover the Surprise!," to run through June 10.

Working with hand puppets and her guitar, Linn proves a charmingly informal artist to begin the series. Her rapport with the children is immediate, rather like a visit from a young godmother who asks get-acquainted questions and elicits immediate, loud responses from the huddled children around her. She smooths the way for her puppets by an informal sing-along session.

She introduces her puppets, Catalion, Audrey Duck and Timberlaine Wolf. A skilled ventriloquist, Linn creates differences between them-saucy, spirited and spunky. After the session she will do a bit of informal training, an ingratiating postscript.

Performances will be at 10:30 a.m. and 12:30 p.m. Wednesdays through Fridays and on Saturdays and Sundays at 11:30 a.m., 2 and 3:30 p.m. Details at 381-5395.