To realize high-accuracy posture measurement in large scale, a posture measurement system is proposed in this study based on a six-laser multilateral method, mainly composed of six laser tracers and three reflectors-a different number of laser tracers tracks each mirror. But in this state, the system is positive definite, and it cannot achieve the system parameters by self-calibration. To solve this problem, a stepwise calibration method to obtain the system parameters by three-step calibration is presented. Simulations and experiments are carried out to verify the effectiveness of the proposed method. The experimental results show that the posture errors are distributed over [−13.6″, 6.6″] when the measurement range is [5 m, 7 m], which shows that the new measurement system has a high posture measurement accuracy in large scale.