We used a 300nm polycarbonate (PC) membrane as a supported structure and CsI photocathode was evaporated onto PC surface. The thickness of CsI can be controlled between 100 nm to 1 μm. The effects of deposition speed and deliquescence property on morphology and structure were analyzed. The photoelectric conversion properties of CsI films were analyzed with Manson X-ray source. The energy spectral response peak value of photocathode is above 3000μA/W calibrated at Beijing synchrotron radiation facility. The photocathode was applied to an X-ray streak camera on SG III facility and distinct x-ray images were captured.