Abstract: A method for constructing a free trajectory of a ballistic missile at a specified launch angle includes: setting of an initial state of iterations: based on geodetic coordinates of a launch point and a target point, a launch epoch and the specified launch angle, assuming that the earth does not rotate and a flight time is zero; generating a new flight time in a two-body force model by using known quantities and obtained auxiliary quantities; taking a difference between flight times obtained before and after the iterations as a condition for judging convergence; outputting designed parameters of the ballistic missile after the convergence is reached, or performing a differential correction including the J2 perturbation to improve the precision of the trajectory constructed, and taking a position error of the target point as a convergence condition of the differential correction.