Where i can change few dollars at Beijing Capital Airport? Is there is any bank?

Yes, there are several banks in the terminal building that can provide money exchange service. Don't worry. Hereunder is the detailed information:
T1:
Travelex: Domestic Arrival Hall, F1, T1

T2:
ICE:International Arrival Luggage Hall, F2, T2
Greenet Currency Exchange: public area of International Arrival Hall, F1, T2
Hang Seng Bank: International Arrival Luggage Hall, F1, T2
Travelex:public service area of Departure Hall, T2 (in the central area)

T3:
Bank of China: Departure Hall, F4, T3-C (east)
Construction Bank: Departure Hall, F4, T3-C (west)
Agricultural Bank of China: public area of International Arrival Hall, F2, T3-C (west of Exit B)
Bank of Beijing: public area of Domestic Arrival Hall, F2, T3-C (east of Exit A)
Industrial & Commercial Bank: public area of Domestic Arrival Hall, F2, T3-C (west of Exit C)

Hope helpful!