Clear["`*"]
A = (1/44.27)^2;
B = 15/0.0345;
C1 = 139.62959902286144`;
C2 = 31.427701554689126`;
t = 4/3;
xn = 7.5/0.0345;
ContourPlot[
NIntegrate[1/Sqrt[
A*(B - x1)^2 + C1*(E^(x - xn) - E^(x1 - xn)) +
C2*(E^(t*(x - xn)) - E^(t*(x1 - xn)))], {x, x2, x1}] == 1, {x1,
200, 220}, {x2, 200, 220}, FrameLabel -> Automatic]
感觉画出的图像并不完整，请问是什么原因呢？

总之先加点PlotPoints试下。