Through theoretical calculation, the stress and deformation of surrounding rock can be analyzed, which can provide guidance for support design and optimization of soft rock roadway. In this paper, theoretical solutions for both the optimal support pressure and the allowable maximum displacement of surrounding rock are derived based on the Drucker-Prager (DP) yield criteria and the steady creep criterion expressed by the third invariant of deviator stress. The DP criteria with different parameters are compared and analyzed by an engineering example. Then, based on the calculation results, the effects of long-term strength, cohesion and internal friction angle of soft rock on the maximum plastic zone radius and allowable maximum displacement of roadway are discussed. The results show that the optimal support solution of soft rock roadway based on DP criteria can not only reflect the intermediate principal stress reasonably, but also can compare and discuss the influence of different DP criteria on the calculation results. The higher the long-term strength of the roadway surrounding rock is, the smaller the optimal support force is and the larger the allowable maximum displacement is. When the calculated long-term strength of soft rock can ensure that the deformation of the roadway does not exceed the allowable maximum displacement, the roadway can maintain long-term stability without support. With the increase of the cohesion or internal friction angle of soft rock, the radius of plastic zone decreases gradually, and the allowable maximum displacement is reduced by degrees. Through grouting and other means to improve the strength of surrounding rock can effectively reduce the roadway deformation and save support costs.