Luggage for transit traveller

I will travel from Lhasa to Hong Kong via PEK. Due to the schedule change of my flight to PEK, I will need to stay in Beijing for one night before taking the flight to Hong Kong the next morning. Do I need to pick up my luggage while exiting PEK and check in my luggage when taking the flight to Hong Kong? I am using Air China for both legs of the trip.