Abstract: This paper introduces the miniature soft X-ray detector for detecting the subnanosecond X-ray pulse. Its response time is analyzed and calculated in theory. According to the results of the theoretical analysis, the HFSS based on the finite element theory is used to model and simulate the X-ray diode and the transmission line theory is applied to design the ultra-broad band coaxial line also. The high dielectric strength film is chosen to fabricate the big storage capacitance and therefore to solve the problem that it is difficult to fabricate big storage capacitance and high voltage resistance at the same time. The characteristic of the detector is experimentally examined on the archetype of “Shenguang Ⅲ Laser Facility” and the response time is about 51ps. also, Some different physics aspects that need to be considered when designing such fast response detectors are discussed.