CdZnTe detectors play an important role in international astronomy research. In this paper, the spectra response of CdZnTe planar detector for γ-ray was numerically simulated using GEANT4 Monte-Carlo simulation toolkit, and the influence of the factors, such as transport properties of electrons and holes, applied bias voltage, thickness of detector and other factors were studied. The calculated results show that when the charge collection efficiency (CCE) of electron is high, the dependence of energy resolution on the ratio of electron and hole mobility-lifetime product ((μτ)e/(μτ)h) is strong, and the resolution increases with the decrease of the ratio. The energy resolution and CCE of carrier can be improved by raising up the bias. When the CCE of electron is high, increasing thickness can weaken the hole signal contribution to the total induced charge, and improve the energy resolution. The value of (μτ)eE/d can be used to evaluate CCE of planar CdZnTe detectors for rays of low energy, and the corresponding relationship was calculated.