We present a new method for building algorithms for the simulation of Rutherford backscattering spectra, based on a time-reversal of the ion trajectories after collision. The considerable simplification resulting from this method is an advantage in and by itself and saves a noticeable amount of computation time. After recalling the main features of classical algorithms, we explain the very simple principle of our "retrograde" method and describe the resulting algorithm. We discuss its advantages concerning rapidity, accuracy, simplicity and versatility.