In this paper, we propose an adaptive controller for virtual passive biped dynamic walking on unknown uneven terrain. The adaptive controller consists of a trajectory tracking control law developed via backstepping method to mimic reference passive gait, and a slope estimator for the inclination angle of the terrain. In addition, a re-planning approach is introduced to correct the robot state off-track from the reference gait due to the terrain changes. The controller is validated through the simulations on the mixed uneven terrain consisting of varying slopes and steps. The results suggest that the controller shows comparable cost of transport and greater adaptability to terrain changes compared with certain existing methods.