Aiming at less research on the sunshine temperature field of steel box girder at the middle and western areas of and western China，the temperature field of the steel box girder in the operation stage of the Tuojiang 4th bridge in Luzhou，Sichuan Province was monitored for a long time，the temperature field was then calculated and analyzed using the ANSYS software. The calculated value was compared with the measured data. Based on the measured data of maximum temperature difference between roof and bottom of operational steel box girder in summer，the exponential function was used to describe the sunshine temperature gradient distribution of steel box girder. The Gaussian mixture model was used to describe the probability distribution of the maximum daily positive temperature difference of the steel box girder in summer，and the standard value of vertical positive temperature difference was then calculated. The results show that the calculated temperature agree well with the measured data，and the trend of temperature changing with time is basically the same. Consequently， the accuracy of the finite element model is verified. The standard value of vertical positive temperature difference is 18.4 ℃ obtained by statistical method. The value is slightly higher than that wascalculated by linear interpolation according to general code for design of highway bridges and culverts. Finally，the vertical positive temperature gradient model of operational steel box girder was obtained. The model can be used for the taking values of vertical temperature gradient of steel box girder bridge in Luzhou and its surrounding areas，and the data can be supported for the revision and improvement of relevant provisions on temperature gradient.