Reservations open 18 days, or 20 days (for online bookings) before departure for all long-distance express trains. We recommend you to book the train tickets as soon as the reservation opens.
You cannot buy tickets before reservations open. The booking horizon increased from 10 days to 18 days for stations, and from 12 to 20 days for online bookings, as from 1 January 2013. Read more about China trains.