Many alternative theories have been proposed, among which the scalar-tensor theory is the most interesting one, because it is the simplest and most natural way to modify general relativity, and stands all kinds of tests. With the ever-increasing precision of experiment and astrophysical observation, the second post-Newtonian approximation of relevant theories of gravity are needed, and some observed effects to that order are also needed. We use the metric coefficients and the equation of motion obtained in the second post-Minkowskian approximation of scalar-tensor theory to derive the trajectories of mas-sive particles to that order for the unbounded case. The deflection angle is obtained and compared with previous works.%标量张量理论是目前为止最成功的相对论性引力理论之一,经受住了已有实验和观测的检验.随着实验和观测精度的提高,理论将得到进一步的检验.为此将该理论的可观测效应计算到二阶后牛顿近似非常必要.标量张量理论的二阶后牛顿近似结果已经由Xie等人给出.为得到有质量粒子在弱引力场中的偏转情况,利用二阶后闵可夫斯基近似下的度规解得到了粒子的测地线方程,并通过迭代的方法求得在非束缚条件下,粒子在弱引力场中的轨迹的解析解.进一步利用此结果计算了粒子在弱引力场中的偏转角(与光线的偏转角不同)并与前人的结果进行了比较.
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