ClearAll["Global`*"]
Solve[{(4 Subscript[U, dc])/(\[Pi] Sqrt[
2]) == (1/(I Subscript[C, ft1] \[Omega]) +
I Subscript[L, ft1] \[Omega]) Subscript[I, in1] -
1/(I \[Omega] Subscript[C, ft1]) Subscript[I, T1],
0 == -1/(I \[Omega] Subscript[C, ft1]) Subscript[I,
in1] + (1/(I \[Omega] Subscript[C, T1]) +
I Subscript[L, T1] \[Omega] + 1/(
I \[Omega] Subscript[C, ft1]) +
I \[Omega] Subscript[M, T1R1] Subscript[I, R1]) Subscript[I,
T1], 0 == (1/(I \[Omega] Subscript[C, R1]) +
I Subscript[L, R1] \[Omega] + Subscript[R, 1] -
I \[Omega] Subscript[M, T1R1] Subscript[I, T1]) Subscript[I,
R1], (4 Subscript[U, dc] I)/(\[Pi] Sqrt[
2]) == (1/(I \[Omega] Subscript[C, ft2]) +
I \[Omega] Subscript[L, ft2]) Subscript[I, in2] -
1/(I \[Omega] Subscript[C, ft2]) Subscript[I, R1],
0 == -Subscript[I, in2]/(
I \[Omega] Subscript[C,
ft2]) + (1/(I \[Omega] Subscript[C, T2]) +
I \[Omega] Subscript[L, T2] + 1/(
I \[Omega] Subscript[C, ft2]) +
I \[Omega] Subscript[M, T2R2] Subscript[I, R2]) Subscript[I,
R1], 0 == (1/(I \[Omega] Subscript[C, R2]) +
I \[Omega] Subscript[L, R2] + Subscript[R, 2] -
I \[Omega] Subscript[M, T2R2] Subscript[I, T2]) Subscript[I,
R2]}(*kvl方程*), {Subscript[I, in1], Subscript[I, in2], Subscript[
I, T1], Subscript[I, T2], Subscript[I, R1], Subscript[I,
R2]}] //. {Subscript[C, ft1] -> 1/(\[Omega]^2 Subscript[L, ft1]),
Subscript[C, ft2] -> 1/(\[Omega]^2 Subscript[L, ft2]),
Subscript[C, T1] ->
1/(\[Omega]^2 (Subscript[L, T1] - Subscript[L, ft1])),
Subscript[C, T2] ->
1/(\[Omega]^2 (Subscript[L, T2] - Subscript[L, ft2])),
Subscript[C, R1] -> 1/(\[Omega]^2 Subscript[L, R1]),
Subscript[C, R2] -> 1/(\[Omega]^2 Subscript[L, R2]),
Subscript[R, 1] -> (
8 Subscript[M, T1R1] Subscript[R,
L])/(\[Pi]^2 (Subscript[M, T1R1] + Subscript[M, T2R2])),
R2 -> (8 Subscript[M, T2R2] Subscript[R,
L])/(\[Pi]^2 (Subscript[M, T1R1] + Subscript[M,
T2R2]))} // FullSimplify

"碰到无穷表达式 1/0."
"在本次计算中，\!\(\*StyleBox[\"Power\", \"MessageName\"]\)\!\(\*StyleBox[\"\
::\", \"MessageName\"]\)\!\(\*StyleBox[\"infy\", \"MessageName\"]\) \
的进一步输出将被抑制"

不管怎样，你先把那些下标换掉我再看