No, if you booked already, they won't rebook you for free IN ADVANCE (unless the fare is changable)
However, since you seem not to mind staying in PEK for an overnight, Air China will provide a hotel (and meals) should you missconnect. And obviously rebook your flight.
I do not second moondog, by the way - 70min in PEK is certainly tight, but perfectly doable. INT-INT, it is.
Just head straight towards international transfers, and in case of queues, make yourself being noticed. They got screens showing flights closing soon - those people will be let ahead.
Unless the incoming flight is delayed more than 30min or so, you'll make your connection IF no queues or you do whatever reasonable there can be done to be let in
#1 . If it's an on time arrival, you might even have time for the quick visit to the lounge (should your status/travel class allow for that)