We introduce MaskEditor, an object-level 3D neural field editing method based on text instructions. Different from manipulating the whole scene, local editing needs accurate locating and proper field fusion to provide a realistic object-level replacement. We utilize a 3D mask grid to accurately localize the target object leveraging the 2D segmentation information provided by the Segment Anything Model (SAM). The whole scene is divided into the object field and background field based on the learned 3D mask. Subsequently, we apply the Variational Score Distillation (VSD) to optimize the object field and leave the background field unaltered, which achieves editing results aligned with text instructions. Furthermore, we implement composited rendering and coarse-to-fine editing strategy to enhance the editing quality and the consistency of the edited object with the original scene. Qualitative and quantitative evaluations confirm that MaskEditor achieves more precise and superior local editing compared to baselines.