We described the design of the measuring system for soft X-ray absolut
e intensity in detail. The system consists of two parts: the ionizatio
n chamber and the monitoring (or, to be calibrated) detector and its t
ransfering system. The system can be used as the primary standard dete
ctor for the measurement of soft X-ray absolute radiation flux in the
energy range from 50 to 2000 eV after calibrated. The uncertainty of t
he system is also given.