Do not understand 栅源栅漏的总电容 your mentioned, please specify it in English.
What it sounds like to me is your would like to simulate the gate capacitance + field capacitance.
The later one (i.e. 边缘场电容) require the definition the environment condition - air, which is excluded in the expression of "@V(Drain)/@I(Drain)". It is straightforward.
However, the trick thing is your device structure. MEDICI is very likely to give you weird value if it is not tuned precisely.
Also error might occur to your expression " extract name=R express="@V(Drain)/@I(Drain) ", which might be better to be changed into