The landing safety and accuracy of a Mars lander will be seriously degraded due to multiple unknown disturbances or perturbations in the powered descent phase. A novel composite guidance algorithm is proposed to improve the landing performance in this paper. The presented guidance algorithm, with a composite hierarchical framework, is developed by the combination of disturbance observer-based control and multiple sliding surfaces guidance theory. The major disturbance owing to the Mars wind could be estimated through a disturbance observer and incorporated in the feed-forward compensation in the inner loop, other disturbances or perturbations could be attenuated by the multiple sliding surfaces technique in the outer loop. The composite guidance algorithm could be utilized without a pre-computed reference trajectory and it has enhanced anti-disturbance capability compared with the multiple sliding surfaces guidance law. Its global stability is verified using a Lyapunov-based approach. Monte Carlo simulation results show that the composite guidance law has a better performance on guiding a Mars lander from the point of engine ignition to the desired landing point in the presence of disturbances and perturbations.