The influence of unvented hood on the initial compression wave generated by a high-speed train entering a tunnel is investigated using computational fluid dynamics. Comparisons with experimental data are first carried out to verify the numerical model. The relationship between the pressure gradient peaks and main aspect factors is studied by parametric analysis. Influences of train speed, blockage ratio of train to tunnel, section area ratio of hood to tunnel, and hood length are investigated. Based on the numerical results, two empirical formulations are proposed to predict the influence of hood and tunnel geometries on the maximum pressure gradient during the CRH3 entering a tunnel with unvented hood.