Abstract: It will be helpful for soft X-ray Spectrometer that X-Ray diode (XRD) can be improved to be more compact. And it will be convenient that other X-ray diagnosis apparatus integrate the mini - XRD. A new XRD with smaller cathode was designed, which owned more reasonable capacitor for power storage and smaller size. Microwave performances of the output part were simulated. The results show that signal distortion and reflection is ignorable in the range of 0 -20 GHz and experiments requirements can be satisfied. And detector character test experiments were carried on the short pulse laser equipment. Data indicates that capability of temporal resolution was improved by 20%. Because of the study of detector with smaller cathode area, XRD will be more popular for estimating of X-ray flux.