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The energy-saving and green environmental protection magnesium oxychloride cement (MOC) is introduced into the pavement base and subbase to improve the shortcomings of CO2 emissions and high industrial energy consumption in the production process of traditional cementitious materials such as lime and Portland cement. Box−Behnken design of design-expert is employed for experiment arrangement, in which MgO/MgCl2 molar ratio, MOC content, and fly ash content are influencing factors, while response values are 7d unconfined compressive strength (USC) and 1d softening coefficient (SC) of solidified soil. The response surface methodology (RSM) is used to optimize the ratio of three additives, and the effects of various factors on the response value are investigated by response surface model analysis and interaction analysis. The results show that the MOC content has the most excellent effect on 7d unconfined compressive strength, and the mutual influence for the MOC content and fly ash content are significant, respectively. However, the influential factor is the fly ash content for the 1d softening coefficient. It is predicted by the RSM analysis that the optimum balance of USC and SC is 8.61 for the MgO/MgCl2 molar ratio, 18% for the MOC content, and 20.36% for the fly ash content. With the additives in the optimal ratio, the actual unconfined compressive strength and softening coefficient of stabilized soil are 2.56 MPa and 0.76, respectively. It is confirmed that the response surface methodology plays an important part in optimizing the proportion of MOC-stabilized clayey soil.