In order to enhance specific on-resistance (Ron, sp), the trench gate structure was also introduced into 4H-SiC MOSFET as Si MOSFET. But the 4H-SiC trench gate has worse off-state characteristics than the Si trench gate due to the incomplete gate oxidation process (Simonka et al., 2017). In order to overcome this problem, P-shielding trench gate MOSFET (TMOS) was proposed and researched in previous studies. But P-shielding has to be designed with minimum design rule in order to protect gate oxide effectively. P-shielding TMOS also has the drawback of on-state characteristics degradation corresponding to off state improvement for minimum design rule. Therefore optimized design is needed to satisfy both on and off characteristics. In this paper, the design parameters were analyzed and optimized so that the 4H-SiC P-shielding TMOS satisfies both on and off characteristics. Design limitations were proposed such that P-shielding is able to defend the gate oxide. The P-shielding layer should have the proper junction depth and concentration to defend the electric field to gate oxide during the off-state. However, overmuch P-shielding junction depth disturbs the on-state current flow, a problem which can be solved by increasing the trench depth. As trench depth increases, however, the breakdown voltage decreases. Therefore, trench depth should be designed with due consideration for on-off characteristics. For this, design conditions and modeling were proposed which allow P-shielding to operate without degradation of on-state characteristics. Based on this proposed model, the 1200 V 4H-SiC P-shielding trench gate MOSFET was designed and optimized.