Terminal 3 of Beijing Capital International Airport might be the second largest airport passenger terminal building of the world (DXB T3 being the reigning champion), but it's apparently not enough to absorb the amount of traffic.
Last year, 83.7 million passengers traveled through the national airport, which is designed to handle 76 million passengers.
Up to 72 million passengers, 2 million tons of cargo and 620,000 planes by 2025. That's what the new airport will handle with its 4 runways (though more are rumored). Situated south from the capital (PEK is northeast), it will clearly alleviate the congestions problems, though it remains unclear how the division of traffic will happen.
Although called temporarily named after the capital (Beijing Daxing International Airport or Bejing Capital Second International Airport), it will also serve the city of Tianjin (about 15 million inhabitants) which airport now handles around 11m passengers, but is expected to handle between 30 and 40 million by 2020.
I've quickly laid out a map of the various airports around the capital for your understanding: