[Purposes] This paper carried out research on temperature difference value in vertical temperature gradient of steel-concrete composite beams with PK section without pavement during construction to provide a scientific basis for engineering design and construction. [Methods] Finite element numerical simulation and a real bridge test in temperature field were conducted on the composite beam of a bridge in the construction stage in Jiangmen City, Guangdong Province. Based on meteorological parameters on sunny days and cloudy days with long sunshine duration in summer, numerical simulation was carried out to obtain daily maximum vertical positive temperature difference data. Statistical analysis was made on simulation data, and an exponential function was used to describe the vertical positive temperature gradient distribution curve. Using three meteorological parameters of latitude, elevation, and the diurnal temperature range corresponding to the annual maximum temperature, the paper proposed a prediction formula for the standard value of vertical positive temperature difference. Finally, zoning of temperature difference value was conducted according to the predicted results for 235 cities across China. [Findings] The numerical results agree well with the measured data. The daily maximum vertical positive temperature difference data obey the generalized extreme value distribution. Taking the 50-year return period as the standard, the standard value of vertical positive temperature difference is 14.946 °C. The proposed prediction formula can accurately reflect the change law of the standard value of vertical positive temperature difference. According to the prediction results, China is divided into 3 regions for temperature difference values. The values for the three zones are 14 °C, 16 °C, and 18 °C. [Conclusions] The proposed prediction formula for the standard value of vertical positive temperature difference and zoning of temperature difference value are suitable for composite beams with PK section without pavement. This research provides a reliable reference basis for engineering design and construction.