i booked online train ticket from beijing to guangzhou ,how can pickup at the capital airport

You can collect your ticket at the railway ticket counter in the airport. Here are their locations:
1. Arrival floor, opposite to No.9 gate of Terminal 2;
2. Counter No.22 of L Check-in Area in the Departure Hall on the 4th floor of T3.

From March 1, 2014, the official railway ticket website, 12306.cn, carries out identity verification, which is aimed to improve real-name ticket system and crack down on ticket scalpers. Foreign passengers have to have their identities checked at ticket counters in railway stations by presenting valid passports before collecting paper tickets. Ticket offices in cities and airports do not offer identity verification service. Thus, tickets booked online can only be picked up at railway stations.