\[Theta]g = 20 \[Degree];
\[Alpha] = 30 \[Degree];
d = 30;
m = 50;
l = 50;
n = 1;
\[Theta]m = ArcCos[Cot[\[Alpha]]*Tan[\[Theta]g/2]];
p13 = ListPlot3D[{{0, l + d/2*Cot[\[Alpha]],
2*m*Sin[\[Theta]g]}, {-d*Sin[\[Theta]m] + 0,
l - d/2*Cot[\[Alpha]],
2*m*Sin[\[Theta]g] +
d*Cos[\[Theta]m]}, {0, (l + d/2*Cot[\[Alpha]] +
m*Cos[\[Theta]g]), m*Sin[\[Theta]g]}, {-d*Sin[\[Theta]m] + 0,
l - d/2*Cot[\[Alpha]] + m*Cos[\[Theta]g],
m*Sin[\[Theta]g] + d*Cos[\[Theta]m]}}, Mesh -> None]

竖轴范围太小了