ABSTRACT Heavy‐duty robots such as hexapod robots exhibit tremendous potential with their high payload ability and terrain adaptability. This paper introduces a control approach for heavy‐duty hexapod robot that traverses uneven and transition stage from a flat surface to a sloped terrain without prior geometric knowledge of the environment. Using force sensors on the robot feet and position sensors at its joints, the transition from a flat surface to a sloped terrain is detected. Constraint models based on terrain geometric information and joint limit positions are presented. The models are derived to estimate the unknown slope gradient based on the landing feet positions. The constraints for different terrains have also been investigated. An optimal force allocation method under contact constraints is developed to minimize the linear and quadratic costs in the commands and constrained contact forces. Extensive experiments have been conducted, and the results have demonstrated the effectiveness of the proposed control approach.