As a novel low-cost semiconductor with extraordinary photoelectric property, the inorganic CsPb X ₃perovskites have become emerging materials for the next generation of X-ray detectors in the past decade. However, most of recent studies of CsPb X ₃perovskite X-ray detectors are based on their internal photoelectric effect. Though it is also important and widely used in vacuum X-ray detectors, the external photoelectric effect of CsPb X ₃perovskite has been rarely studied by now. Thus, the response sensitivity of the CsPb X ₃perovskite’s external photoelectric effect in the X-ray region is studied in the present paper. First, a 230-nm-thick CsPbI ₂Br membrane is prepared on a metal substrate by a conventional one-step deposition method, with a precursor solution used. Then the external photoelectric responsivity and quantum efficiency of the CsPbI ₂Br membrane are calibrated in a range from 2000 to 5500 eV at Beijing Synchrotron Radiation Facility. The responsivity is over 5.1 × 10 ^–5A/W in the range and the quantum efficiency is over 23%. These calibration data are close to those of a traditional X-ray photoelectric material CsI. The Monte-Carlo method is utilized to simulate the external photoelectric effect of CsPbI ₂Br perovskite, and the external photoelectric responsivity is calculated. The calculated data match well with the calibration, proving the Monte-Carlo method feasible for the external photoelectric effect simulation of CsPb X ₃perovskite. Then the external photoelectric responsivities and quantum efficiencies of CsPb X ₃perovskites are calculated via the Monte-Carlo method in the X-ray range from 2000 to 10000 eV. The calculated responsivities of different CsPb X ₃perovskites are all close to the responsivity of CsI, and an order of magnitude higher than that of Au, and the CsPb X ₃quantum efficiencies also follow a similar scenario. This indicates that CsPb X ₃perovskites have good external photoelectric properties and potential applications in X-ray vacuum detectors such as photocathode and photomultiplier. The influence of thickness on CsPb X ₃photoelectric response is also studied in this paper via Monte-Carlo simulation. The results show that the responsivity increases with the material thickness increasing, which is due to the increased X-ray absorption. The responsivities all reach their upper limits at a material thickness of about 150 nm, which means that the electrons generated at 150 nm can hardly escape from the material surface. It is indicated that the thickness of CsPb X ₃should be no less than 150 nm to obtain the optimal photoelectric response.