We designed a simplified X-ray diode (XRD) to detect soft X-ray emission, which has advantages of simplified structure, bigger response area, and lower requirements for bias voltage and vacuum level. The response time and saturation current were analyzed. Meanwhile, the formula to calculate the energy was established and the XRD was calibrated at synchrotron radiation source. The energy calibration result matches the theoretical calculation one. With this XRD, we measured the pulse width and energy of discharged-pumped Ne-like Ar soft X-ray laser of 46.9 nm.