MOS field effect transistor(FET) with Dog Bone shape gate is a symmetric layout structure with hardened radiation-total-dose effect (RTDE). In order to calculate its equivalent aspect ratio, it is divided into a regular MOS FET (host MOS) and two parasitic MOS FETs (edge MOS) in parallel. The model of this device is constructed in Silvaco TCAD software, then a fitted formula of the equivalent aspect ratio is derived based on the simulation of drain current with respect to the width and length of host MOS, and overlap width between polysilicon and active area of edge MOS. The comparison between the fitted formula and measurements of sample transistors taped out in CSMC 0.5 μm DPTM CMOS mixed-signal process indicates the fitted formula for equivalent aspect ratio of Dog Bone Gate MOS FET matches the measurement.