Abstract: Aiming at the difficulty in detection of tree-contact single-phase-to-ground faults（TSFs）, the modeling and detection method for TSFs are studied. By analyzing temperature changes of trees during faults, a time-varying model for transition resistance of TSFs is established based on actual physical parameters of trees, i. e., resistivity, height, radius, and resistivitytemperature characteristics. Using this model, the TSF characteristics of zero-sequence voltage and current variation with time for different 10 kV systems are analyzed and concluded. Based on the characteristic that TSF zero-sequence current rises slowly and monotonously, a TSF detection method is proposed which monitors the time duration of the zero-sequence current exceeding the threshold, and the measurement reliability is improved by using multi-cycle measuring data. The model and the method are verified with digital simulations and 10 kV TSF experiments. The verification results indicate that the model can accurately simulate the slowly time-varying curves of transition resistance for TSFs caused by size difference of trees, and the detection sensitivity of the proposed method can reach tens of kiloohms, which can greatly improve the detection ratio of TSFs.