Abstract: The temperature change caused by hydration leads to early-age cracking in concrete box girder. The early-age cracking risk is further improved with low air temperature and large daily temperature difference, especially in Northwest China. To fill this gap, a temperature experiment and numerical simulation were performed on an actual concrete box girder segment in Northwest China. The temperature field, thermal stress and cracking risk were analyzed using evolution curves, distribution curves and contours. The key parameters that influence the hydration heat temperature, including the cement hydration heat release, cement content, height-width ratio of web, initial temperature, convective coefficient of top plate surface, were analyzed. An anti-cracking case based on parameters analysis was put forward. The results indicated that the temperature evolution can be divided into three stages: warming, cooling and environment significantly impacting. Along the thickness of each plate, temperature distributed is single peak in the center. Along the width or height, temperature distributed is double peaks at axillary position for the top and bottom plates, and single peak in center for the web. The axillary position and web have high thermal stress and significant cracking risks. The temperature difference of each plate, and the early-age cracking risk can be reduced by effectively adjusting the key parameters. Among these, the former two parameters are the most significant factors. The maximum cracking risk can be decreased by 15.7% for every 50 kJ/kg hydration heat reduction. The maximum cracking risk can be decreased by 13.1% for every 50 kg/m³ cement content reduction.