Considering the requirement of system design on evaluating the accuracy of Helmet Mounted Sight (HMS), it is necessary to determine the attitude of a helmet in 6 Degree of Freedom (DOF). All parameters regarding to the helmet characteristics such as activity range, maximal Field of View (FOV), position and direction of sight line, relevant attitude variations, etc, need to be calculated accordingly. Two cameras are mounted in the cabin of a helicopter, and images acquired from different directions are processed simultaneously. A mathematical model is derived and a monocular measurement method is implemented through the introduction of pyramid principle, space resection algorithm and attitude detection method based on exterior orientation elements. Based on the data fusion of multi-Sensor, optimal solutions are obtained by means of the further studies on processing the helmet pose data.