The one-hour live-fire exercise at Pocheon, 30 kilometres (20 miles) south of the tense land border with North Korea, would be the largest ground-air joint fire drill this year, an Army spokesman said.
It was to begin at 2:00 pm (0500 GMT), he said.
Military officials said 105 types of weaponry -- including tanks, anti-tank missiles, attack helicopters, self-propelled guns, multiple launch rockets and six fighter jets -- would be mobilised at Pocheon.
The Army spokesman said it would be the first time multiple launch rockets had been mobilised in such a joint fire drill.
The Navy is also conducting a firing exercise off the east coast.
The South says its drills are defensive, but tensions have been high on the peninsula since the North shelled a South Korean island near the contested western sea border one month ago.
The North said its shelling was in response to a live-fire drill on Yeonpyeong island. The South said it had been staging such artillery exercises for 37 years and the North was seeking a pretext to attack.
Seoul staged a repeat drill on the same island on Monday, backed up by jet fighters and warships, but the North did not follow through with threats to hit back.
Some analysts said Seoul's show of force deterred the North. Others said the hardline regime had been told by close ally China to exercise restraint before a visit to Washington by President Hu Jintao on January 19.
The North's official news agency on Thursday branded the South's military as "warmongers".
It described the four-day naval exercise as "fanatical drills for invasion of North Korea" and accused Seoul of trying to disguise the "aggressive characteristics" of the drills.
By North Korea's standards, the wording was relatively mild. In another sign that tensions are easing, the South's Joint Chiefs of Staff said it had lowered a military alert issued for frontline areas before and during Monday's drill.
The United States, which has 28,500 troops based in the South, warned North Korea there was no reason for it to respond to the latest drills.
White House spokesman Robert Gibbs said the manoeuvres had been announced well in advance and were transparent and defensive, and "should in no way engender a response from the North Koreans".
The Korea JoongAng Daily newspaper said that while the North failed to respond to Monday's drill, the two militaries were jockeying for position behind the scenes.
The paper, quoting Seoul military sources, said the North had made preparations to fire anti-aircraft missiles.
Light beams to guide them were detected at an anti-aircraft base in the North's Hwanghae province but no missiles were launched, it said.
A Defence Ministry spokesman declined comment.