The problems of difficult spoil discharge and cutterhead mudding pose many challenges to the advancement of earth pressure balance (EPB) shield tunneling, especially for high-viscosity shale strata, where spoil improvement is necessary. Relying on the shield tunneling project of Shuan Tunnel (Sichuan section) of Chengdu-Chongqing Central Line, foam performance evaluation, vane shear test, interface shear test, and indoor shield tunneling model tests were carried out on shale to explore the optimal concentration range of foaming agent, and the effects of water content and foam injection ratio on the normal interface force, longitudinal interface force, and cutterhead mudding were investigated. The results show that: (1) The optimal volume fraction of the foaming agent is around 3%, with the corresponding foam expansion ratio and half-life being 29.1 and 8.71 min, respectively. (2) Based on the above optimal volume fraction, with the increase of water content and foam mass ratio, the vane shear strength, direct shear strength, cohesion, internal friction angle of shale, the average pressure of jacks, and the torque under the action of unit jack pressure all decrease to varying degrees. The adhesion amount of spoil on the shield cutterhead does not change significantly, but the consistency of shale decreases, and the cutterhead mudding is improved. (3) Overall, the average advancement speed increases. The greater the average pressure of the jacks, the greater the cutterhead torque. The improvement effect of the foaming agent on shield tunneling is better than that of water content. The research results have certain reference significance for evaluating the construction of EPB shield tunneling in high-viscosity shale strata.