Abstract: Based on the genetic algorithm (GA), a direct 3-dimensional algorithm is proposed for low-altitude-penetration trajectory under conditions of complex terrain and threat. Optimal trajectories are given by optimizing the original code group(chromosome), objective function, and genetic operators such as reproduction, crossover and mutation. Coordinate direct code design, super gene protection, distributed search and guide variation are adopted. Simulation results proved that the method is logical, feasible and effective, and attained real time effect.