User needs vary significantly across different tasks, and therefore their queries will also vary significantly in their expressiveness and semantics. Many studies have been proposed to model such query diversity by obtaining query types and building query-dependent ranking models. To obtain query types, these studies typically require either a labeled query dataset or clicks from multiple users aggregated over the same document. These techniques, however, are not applicable when manual query labeling is not viable, and aggregated clicks are unavailable due to the private nature of the document collection, e.g., in personal search scenarios. Therefore, in this paper, we study the problem of how to obtain query type in an unsupervised fashion and how to leverage this information using query-dependent ranking models in personal search. We first develop a hierarchical clustering algorithm based on truncated SVD and varimax rotation to obtain coarse-to-fine query types. Then, we propose three query-dependent ranking models, including two neural models that leverage query type information as additional features, and one novel multi-task neural model that is trained to simultaneously rank documents and predict query types. We evaluate our ranking models using the click data collected from one of the world’s largest personal search engines. The experiments demonstrate that the proposed multi-task model can significantly outperform the baseline neural models, which either do not incorporate query type information or just simply feed query type as an additional feature. To the best of our knowledge, this is the first successful application of query-dependent multi-task learning in personal search ranking.