The relationship between the photon flux and the wave length of the soft X - ray beamline in Hefei Light Source (HLS) was calculated. The photocurrent was measured by an AXUV photo diode when a condense zone plate of CZP32 type and a pinhole of 30 microns were adopted by the monochromator. Accoding to the photocurrent and the quantum efficiency of the diode
the absolute photon flux outside the vacuum separation window in a range of 2. 8 nm to 3. 9 nm was measured to be 2. 7 ×105-8. 0 ×105 photons/(s. mA·0. 1nmBW). By theoretical calculation
It was found that the photon flux inside the vacuum chamber was about 107 photons(s·mA·0. 1nmBW)
which was two orders higher than the value outside.