Clear["Global`*"]rr = 0.99;
r = Sqrt[rr];
fsr = 1500;
mod = 30;
\[Phi][w_] := w/fsr;
f[w_] := (r (Exp[I \[Phi][w]] - 1))/(
1 - r^2 Exp[I \[Phi][w]]);
m = mod/fsr
fm1[w_] := (r (Exp[I \[Phi][w + m]] - 1))/(
1 - r^2 Exp[I \[Phi][w + m]]); fm2[w_] := (r (Exp[I \[Phi][w - m]] - 1))/(
1 - r^2 Exp[I \[Phi][w - m]]);
yy = f[w] Conjugate[fm1[w]] - Conjugate[f[w]] fm2[w];
Plot[Im[yy], {w, 0.5, 1.5, 0.00001}, {x, 0.9, 1.1}, {y, -1, 1},
PlotRange -> All,
AxesLabel -> {"\[Phi]", "\[Epsilon]/2\!\(\*FormBox[SqrtBox[\(PcPs\)],
TraditionalForm]\)"}]

你的变量都没x和y 为啥写？试下这个
Plot[Im[yy], {w, 0.5, 1.5},
PlotRange -> All,
AxesLabel -> {"\[Phi]", "\[Epsilon]/2\!\(\*FormBox[SqrtBox[\(PcPs\)],
TraditionalForm]\)"}]

首先你要知道你想画的是个啥。。。
然后再画。。。
Clear["Global`*"];
rr = 0.99;
r = Sqrt[rr];
fsr = 1500;
mod = 30;
\[Phi][w_] := w/fsr;
f[w_] := (r (Exp[I \[Phi][w]] - 1))/(1 - r^2 Exp[I \[Phi][w]]);
m = mod/fsr;
fm1[w_] := (r (Exp[I \[Phi][w + m]] - 1))/(1 -
r^2 Exp[I \[Phi][w + m]]);
fm2[w_] := (r (Exp[I \[Phi][w - m]] - 1))/(1 -
r^2 Exp[I \[Phi][w - m]]);
yy[w] := f[w]*Conjugate[fm1[w]] - Conjugate[f[w]] fm2[w];
Plot[Im[yy[w]], {w, 0.5, 1.5}, PlotRange -> All]
x和y不用特别指定
如果需要，用PlotRange 修改