The CdZnTe (CZT) devices with various screening depth in dimensions 3×3×6mm3 were fabricated. The influence of Ring length (screening depth) and length of crystal on the electric field distribution in CZT devices has been explored by finite element method. The results indicated that ring length (screening depth) plays an important role on the detecting performance of CZT Frisch-ring devices. Longer screening depth gives rise to an electric field which is compressed more greatly. Measured spectra indicated that extreme compressed electric field could reduce detecting resolution. A FWHM energy resolution of approximately 3.71% at 662 keV was obtained for a device with dimensions 3×3×6mm3 and 4.8mm screening depth.