g = 9.8; ll = 1.5; ww = Sqrt[g/ll];
qt = {};
Do[w0 = v0/ll;
s = NDSolve[{q''[t] == -ww^2 Sin[q[t]], q[0] == 0, q'[0] == w0},
q, {t, 0, 10}]; a = (q /. s[[1]]);
qm1 = FindMaximum[a[t], {t, 0.5}];
qm2 = FindMaximum[a[t], {t, 3.3}];
tt = (t /. qm2[[2]]) - (t /. qm1[[2]]);
AppendTo[qt, {qm1[[1]] 180/Pi, tt}], {v0, 0.1, 5.4, 0.1}]

你没贴出图的代码啊

不好意思，我忘了
g1 = ListPlot[qt]

我这里正常出图了