2D and 3D semantic segmentation play important roles in robotic scene understanding. However, current 3D semantic segmentation heavily relies on 3D point clouds, which are susceptible to factors such as point cloud noise, sparsity, estimation and reconstruction errors, and data imbalance. In this paper, a novel approach is proposed to enhance 3D semantic segmentation by incorporating 2D semantic segmentation from RGB-D sequences. Firstly, the RGB-D pairs are consistently segmented into 2D semantic maps using the tracking pipeline of Simultaneous Localization and Mapping (SLAM). This process effectively propagates object labels from full scans to corresponding labels in partial views with high probability. Subsequently, a novel Semantic Projection (SP) block is introduced, which integrates features extracted from localized 2D fragments across different camera viewpoints into their corresponding 3D semantic features. Lastly, the 3D semantic segmentation network utilizes a combination of 2D-3D fusion features to facilitate a merged semantic segmentation process for both 2D and 3D. Extensive experiments conducted on public datasets demonstrate the effective performance of the proposed 2D-assisted 3D semantic segmentation method.