Abstract: In order to explore the theoretical analysis method for the creep effect of prestressed concretion-PC beams under complex variable loads and further clarify the cracking downwarping mechanism of PC beam Bridges, this research firstly summarizes and analyzes the influencing factors, development mechanism and, theoretical prediction model of concrete creep. Based on L. Boltzmann superposition principle, a modified superposition method under the influence of multiple factors is proposed. Secondly, large-scale PC beam model tests under stepped variable stresses are carried out to obtain the creep effect test data of different parts of the model PC beam under different load conditions. Finally, the accuracy of the modified superposition method is verified through the comparative analysis of the test data and of the theoretical calculation results, and the applicability of the modified superposition method to the calculation of the creep effect under continuous varying stress conditions is further analyzed. The research results show that the creep strain of the test PC beam continues to increase under the stepwise increasing and decreasing stresses, and that the creep strain of the test PC beam fluctuates but increases overall under the stepwise fluctuating stress. The creep theoretical results obtained by modified superposition method have the same development rule as the test data under the step-varying stress condition, and the relative error is 5.2%~8.3%, which is small. Under continuously increasing stress conditions, the calculated results of modified superposition method agree well with the theoretical results of superposition method. Under the condition of continuous stress reduction, the error between the modified superposition method and the elastic aging theory is 0.95%. In summary, the modified superposition method established can accurately predict the creep deformation under varying load conditions under the additional influence of practical factors, further reveal the creep mechanism of concrete under complex conditions, and provide technical support for the long-term creep effect analysis and prediction of PC beam Bridges under complex load conditions.