Stacked with the agency's Orion crew capsule, the SLS is 332 feet tall and currently sits at the Kennedy Space Center in Florida.
SLS will launch the next generation of deep space operations, including Artemis missions, on and around the moon.
It was initially slated to take off in November but has run behind schedule.
NASA said in a Friday release that the integrated system is entering the final phase of preparations for an upcoming uncrewed flight test around the moon – including a series of integrated tests.
Its mission, known as Artemis I, will pave the way for a future flight test with crew and NASA aims to send the first woman and the first person of color on the moon in coming years.
"It’s hard to put into words what this milestone means, not only to us here at Exploration Ground Systems, but to all the incredibly talented people who have worked so hard to help us get to this point," Mike Bolger, Exploration Ground Systems program manager, said in a statement "Our team has demonstrated tremendous dedication preparing for the launch of Artemis I. While there is still work to be done to get to launch, with continued integrated tests and Wet Dress Rehearsal, seeing the fully stacked SLS is certainly a reward for all of us."
Testing includes interface verification testing, program specific engineering testing, end-to-end communications testing, countdown sequencing testing and wet dress rehearsal testing.
According to Space.com, in a Friday media briefing, deputy associate administrator for exploration systems development at NASA Headquarters Tom Whitmeyer told reporters that the stacked rocket and crew capsule will roll out to the Launch Pad 398 in late December for testing and west dress rehearsal testing in January before going back for more checkouts followed by another move onto the pad.
"Prior to rolling out to the pad for wet dress, teams will conduct the first of a two-part test of the flight termination system inside the VAB. Once the systems are verified, the 322-foot-tall rocket will roll back into the VAB for final inspections and checkouts, including the second part of the flight termination system test, ahead of returning to the pad for launch.," NASA wrote, noting that Artemis I mission operations teams would continue to run additional launch simulations leading up to launch.
The agency said it would set a specific launch date following a successful wet dress rehearsal.
NASA already fired up the core stage and its engines twice this year to ensure that the rocket's components were working – though the first hot fire test in January ended early due to a hydraulic system issue.
NASA eventually completed the engine test firing of its moon rocket in March.
The 1972 Apollo 17 mission was the last in which humans traveled to the moon.