【求助】为什么图里没有曲线呢？g = 9.8; L = 0.

因为你的代码里没有体现出yo是一个变量，改成这样就好了
g = 9.8; L = 0.852;
tmax[yo_] := Sqrt[2*(L + yo/2)/g];
equs[yo_] := {y'[t] == -Sqrt[((g*(yo^2 + 2*L*yo - y[t]^2 - 2 L*y[t]))/(L + y[t]))], y[0] == yo - 0.0000000001};
s[yo_] := NDSolve[equs[yo], y, {t, 0, tmax[yo]}];
Manipulate[Module[{ysol},
ysol = y /. s[yo][[1]];
Plot[ysol[t], {t, 0, tmax[yo]}]], {yo, 0.1, 0.8}]