A cylindrical gates model of the static induction transistor is proposed and mirror method is used to calculate the distribution of electric potential.The results show that:the potential barrier is directly determined by channel over pinchedoff factor;gate efficiency η decreases as the gate dimension α2 and shifted gate voltage are minished,and what differs from the firstorder theory is that η will tend to zero at the shifted gate voltage tends to zero when VD=0;at low current,the voltage amplification factor μ increases as the drain current rising.When the drain current reaches certain degree,the voltage amplification factor keeps almost constant.In the end,an analytical description of SIT’s characteristic suited to both triodelike and mixed I-V characteristics are obtained.The predicted I-V curves are consistent perfectly with the reported experimental ones.