The new rail lines built at a cost of $3.2 billion would increase the number of subway lines in the Chinese capital to eight and the total length of the track to 200 km from 142 km.
The opening comes a day ahead of the implementation of a broad traffic ban that will keep more than one million cars off Beijing's streets during the August 8-24 Olympics and will add significantly to the burden on the city's public transport system.
Beijing has banned vehicles with odd and even-numbered licence plates on alternate days for two months starting July 20.
Battling air pollution, Beijing has invested massively on easing transport and reducing traffic congestion to encourage the residents to use public transport.
The new lines include one to the airport and another to the Olympic Branch line running 4.5 kms which will carry spectators to the main Games' venues in northern Beijing.
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The third line will run 25 km northwest to southeast, official Xinhua news agency said.
Before and during the Games, the Olympic Branch Line will open exclusively to participants of the Games, including athletes, coaches, journalists and spectators holding the tickets of the day, Zhou Zhengyu, Deputy Head of Beijing Municipal Committee of Communications, said.
He said the new rail lines would increase the system's carrying capacity by 800,000 passengers.
"We are confident that with these new subway lines we can help cope with the extra burden of commuters," he said.
Beijing subway operators have estimated a daily maximum of 30,000 passenger trips during the Games, Liu Jian, deputy head of the Beijing Mass Transit Railway Corporation Ltd, said.