A statistical model, that is the Kratky-Porod (KP) chain model (Kratky and Porod 1949), was first introduced to the pulp and paper field in this paper. Based on this model, a new method for characterizing the bending flexibility of fibers was developed and the pulp demand of the thinnest sheet was evaluated. Firstly, for the three types of pulps, the experimental results were compared with the KP chain model, and the resulting determination coefficients were all above 0.95, which proved that the model was feasible to be applied to these three fibers. Then, the relation between fiber bending rigidity and that of cellulosic chains inside was discussed to deduce the fiber bending flexibility. The results that have certain statistically significant were in the range of 0.6×1011~3.5×1011 N-1m-2, which was comparable to that of other direct methods recorded. Finally, as an application of the model, the critical concentration of paper-forming was evaluated by the fiber dimension quantified in the KP chain model. At this concentration, fiber network in one layer represented the thinnest paper in theory. To form a paper sheet with the area of 1m2, the minimum pulp assumption was in the range of 0.06~0.44g in theory. In conclusion, the KP chain model can be well applicable to describe the morphological characteristics of plant fibers, which will have more potential applications in the pulp and paper field.