In this study, a CNTs-based paper-based sensing device was fabricated via screen printing for determining solution pH. The ink used for printing contained 5 wt% CNTs and SDS surfactant to avoid agglomeration. The screen printing was conducted to define the sensor channel and electrode dimensions on paper, and the resultant CNTs formed a uniform film after thermal annealing. The CNTs formed a porous 3D structure within paper, and the device was found to be flexible even after thermal annealing. The optimal L/W ratio of the CNT sensor was tested to obtain the optimal sensor response and sensitivity, with L/W of 4 being chosen as the optimal to offer a low resistance. The sensors were then tested for their responses to pH changes, with the responses decreasing with increasing pH. The possible mechanism for this phenomenon was attributed to the increasing OH-group doping effect on the CNTs. The study showed that the CNTs-based paper-based sensing device fabricated via screen printing is a promising method for determining solution pH and can be optimized for desired results.