In response to the problem that steel box beam bridges that have been in operation for 8 to 10 years in the Urumqi region have developed cracks, temperature monitoring was performed on unpaved steel box beam bridges in the Urumqi region of Xinjiang, which was a region with a large temperature difference in China. On the basis of the temperature monitoring data, the temperature gradient distribution pattern of the steel box beams when the temperature difference between the top and bottom plates under solar radiation maximized was presented by a function fitting method. A statistical method was chosen to analyze and calculate the standard value of the most unfavorable temperature difference of the steel box girder solar radiation for the 100 year return period and to obtain the most unfavorable temperature gradient pattern. Based on the obtained unfavorable temperature gradient pattern, the Midas Fea NX finite element simulation software was used to simulate the unfavorable temperature effect. The results show that both transverse and longitudinal temperature gradients exist in steel box girders under insolation. Using the statistical analysis method, the standard value of vertical positive temperature difference for 100 reproduction periods was obtained as 29.8 ℃, which is higher than the value of 26 ℃ of vertical positive temperature difference for unpaved steel box girders obtained by linear interpolation in the current specification in China. According to this temperature gradient mode as the finite element results of temperature loading show that the temperature stress of steel box girder under the action of temperature reaches or even exceeds the stress level of normal design vehicle loading.