The polysilicon p-i-n diode displayed noticeable process-compatibility and portability in advance technologies as ESD protection device. The paper presented reverse breakdown, current leakage and capacitance characteristics for the fabricated polysilicon p-i-n diode. To evaluate ESD robustness forward and reverse TLP I-V characteristics were measured also. Besides polysilicon p-i-n diode string was investigated to further reduce capacitance and fulfill the requirements of tunable cut-in or reverse breakdown voltage. To explain the effects of device parameter, analysis and discussion about the inherent properties of polysilicon p-i-n diode were processed finally.