The invention discloses a method for evaluating temperature gradient modes of combined plate-girder bridges. The method includes arranging temperature measuring points on concrete roofs and steel webs of the combined plate-girder bridges, acquiring temperatures, analyzing the acquired temperatures of the combined plate-girder bridges, drawing gradient scatter diagrams and carrying out curve fitting by the aid of least square processes to obtain horizontal temperature gradient curves; simplifying the obtained temperature gradient curves to obtain broken lines which are the temperature gradient modes; acquiring daily extremes temperature difference values of various break points of the steel webs in vertical temperature gradient modes and daily extremes temperature difference values of various break points of the concrete roofs in the horizontal temperature gradient modes; statistically analyzing the daily extremes temperature values of the various break points of the concrete roofs and the steel webs of the combined plate-girder bridges to obtain temperature difference probability distribution histograms and carrying out probabilistic fitting on the histograms to obtain daily extremes temperature difference probability density functions; computing temperature difference standard values of non-laying and post-laying steel box girders in different recurrence periods at the step (6) and acquiring suggestive values of the temperature difference standard values.