Large indoor scenes usually have similar structures in the elevation direction, which leads to the degradation of the features of lidar scanning point clouds in the elevation direction, and the traditional lidar SLAM is prone to the mismatch of elevation features. In view of this, a decoupling SLAM algorithm for airborne 3D lidar based on inertial/altitude sensor information assistance is proposed: altitude sensor and inertial attitude are introduced into the point cloud initialization process to improve the initial pose matching accuracy; Decouple the point cloud registration algorithm based on multivariate normal distribution in horizontal and height channels, restrict the direction of point cloud registration, and improve the positioning accuracy in the environment of elevation degradation; At the same time, the traditional SLAM six-dimensional pose solution is reduced to three-dimensional, which reduces the amount of calculation. The cabin simulation scene is built by Gazebo, and the proposed method is verified. The results show that the proposed method can improve the positioning accuracy of lidar SLAM under the degradation of elevation features, which is more than 40% higher than the traditional algorithm, and effectively improves the calculation efficiency.