The calibration experiment of glancing incidence soft X ray planar mirror reflectivity is reported. The Beijing synchrotron radiation facility(BSRF) 3W1B beam line and target chamber with reflectometer was used. Under the specific operation mode of beam current 35 mA～110 mA, the storage ring electron energy 2 GeV, photon energies 50 eV～850 eV. 50 eV～850 eV energy region was divided into four energy sections. The calibration experiment for reflectivity of 5° grazing incidence Ni planar mirror was done. High sensitivity silicon photodiode without dead layer was used as detector replacing X ray diode during the calibration. Therefore,2 to 3 orders of magnitude of the signal noise ratio are increased, and the calibration region is expanded from 150 eV～270 eV to 50 eV～850 eV. The entire calibration curve for 5° Ni planar mirror reflectivity is given. The values of experiment and theoretical calculation are compared and analyzed.