Abstract: Aiming to provide a basis for establishing reference radiation beyond the ISO 4037-1：1996 standard and calculating the conventional true values of special dose quantities of the radiation field, fluence spectrum of X-ray is obtained using a CdZnTe detector with CAPture electrode. The main drawback of the CdZnTe detector is the low energy tail on the left side of the full energy peak due to insufficient charge collection caused by the small mobility-lifetime product. The CdZnTe detector with CAPture electrode reduces the electric field strength in the vicinity of the cathode through utilizing the extended cathodes, which weakens the effect of hole transport on charge collection efficiency and curbs the low energy tail. However, the charge collection efficiency cannot be calculated using the Hecht equation as the electric field in the detector is no longer uniform. In this paper, the formula for calculating the charge collection efficiency of the CdZnTe detector with CAPture electrode was established based on the Shockley-Ramo principle, and the electric field strength in the detector was simulated with finite element analysis software. Furthermore, Monte Carlo simulation was carried out using Geant4. As a result, the mobility-lifetime product was determined, and the pulse height spectra calculated by Geant4 are consistent with the measured results, laying the foundation for establishing the response matrix of the detector.