<p>The oblique detonation chamber has an extreme high and uneven thermal load due to the fast exothermic combustion process induced by oblique shock wave. The active cooling technology of over-pressure water is an effective thermal protection method with the development of additive manufacturing technology. The realizable <i>k-ε</i> model coupling with Volume Fraction (VOF) model is applied to solve the boiling flow of cooling water in the mini-channels. The phase transition and heat transfer characteristics are systematically investigated under different pressures. The phase transition process is first observed in the position of oblique detonation wave, and the bubbly flow, the slug flow, the annular flow and the churn flow are captured in the characteristic cooling channel when boiling flow reaching steady stage. In the rear section of the channel, the uneven heat flux distribution of combustion chamber brings to the churn flow, which enhances the heat transfer compared to the annular flow. Compared to the atmospheric water, the appearances of churn flow and annular flow are significantly decreased with the increasing pressure, and they would disappear when the pressure is exceeding 0.5 MPa. The bubbly flow and slug flow play a dominant role when the pressure is higher than 1 MPa, showing that the over-pressure water can significantly enhance the cooling efficiency of the oblique detonation chamber. With the rise of water pressure, the time point of the appearance of phase transition is obviously lagged because of the increasing latent heat of vaporization, and the cooling efficiency is significantly increased due to the simpler phase transition in higher pressure, which means the over-pressure water could withstand longer heat load of oblique detonation engine.</p>