Abstract: Low energy (less than 50 keV) photon detector is applied extensively to monitoring exoatomsphere nuclear detonation and investigating astrophysical phenomenon. A low energy X-ray detector modular with Si-PD element was developed. The method was explained for scheming multi-detector system. Its design, detector test and technologic characteristics were described. Under room temperature, the noise equivalent input photon energy is 2.1 keV.