Originally Posted by jiejie

The joint sum total of the above responses = confusion. The following will hopefully make things clearer. The most important thing you can do to expedite your process is to have your PEK-SIN boarding pass issued to you in SFO and have your luggage tagged through to SIN (shouldn't be a problem). When you deplane at PEK and approach Immigration area you will have the following choices which you should exercise in the following order of preference:

1) International Transit/Transfer desk. Normally fast process. Your passport and boarding pass will be looked at but not stamped, then you will be sent down the hall to a security checkpoint then back airside. You may then go to Air China's First or Business Class lounge, per the class of service you will be traveling to SIN. Note that once you are airside, you cannot get out again and there's not that much airside besides overpriced shops so make sure you want all that lounge time. In the event this desk is closed (normally open except at odd hours), go to next choice.
2) TWOV (Transit Without Visa desk). Normally fast process. Passport and boarding pass (or printout of onward itinerary if you don't yet have BP) will be looked at and passport stamped for TWOV. Past this queue, you'll head on the people mover to the front terminal via Baggage Claim (walk through/no pickup if your bags are already tagged through to SIN), Customs Exit (just a quick walk through) and to front/nonsecure side of terminal. There is a better selection of restaurants in this part of terminal 3. Then just follow the signs back to Intl Departures, back on people mover and through Exit Immigration and Security--usually not more than about 15-20 minutes to get through the Exit process, then back airside and to the lounges.
3) If TWOV queue is closed, go to the Special/VIP/APEC queue where they will do the same as choice 2. Normally fast process.
4) Regular queues for Foreigners. Normally for those entering China on visas. Can be slow if there are many flights arriving at same time. Do not use this unless all other options are closed (a situation I've never seen at PEK).