I arrive at 7.00am in Beijing airport, would like to visit forbidden city, I have about 7 hours layover

You are suggested to take the airport shuttle bus line 2 to its terminal at Xidan and then walk to the Tiananmen Square. After pass the Tiananmen Square, you can walk to the Forbidden City easily.

Since your arrival time is at around 07:00 and you may easily meet the morning peak time of that city, so taking airport express + subway trains are not suggested. The trains will be so crowded. The airport shuttle bus from the airport to the city proper may take around 1.5 hours during the morning peak.

Thanks for your kind reply, I will do the same on return to airport, Wii keep posted on my trip when done.