For the problem of trajectory modeling in ascent phase of launch vehicle, free-node spline model is proposed to represent the change of this trajectory. The trajectory parameter estimation is converted to spline function coefficient estimation for trajectory determination, thus effectively improving accuracy by reducing the number of estimated parameters. Furthermore, an algorithm for optimization of measurement elements is obtained to improve accuracy of the trajectory determination by using chromosome encoding of measurement elements, constructing fitness function based on weighted ranking and designing the improved proportion selecting operator in basic genetic algorithm frame. Then the combinatorial space of measurement elements is sufficiently searched for optimization. The results of simulation based on the classical trajectory of ascent phase and distribution of stations show that accuracy of ascent trajectory determination is effectively improved. Compared with traditional methods, position determination accuracy of this method is improved from 92.0% to 94.4%. In the case of using free-node spline model, the position determination accuracy is also enhanced to 16.4%~ 88.6%.