Based on mechanism of processing bamboo fiber by combing method, a concept of average combing strength was put forward. Calculation model of relationship between average combing strength of fiber and fiber length and fineness was established with dynamic principle. The model was analyzed and validated, and the results showed that the model of bamboo fiber length and fineness against average combing strength, roller speed and comb-pins parameters was correct. The values calculated for bamboo fiber length by the model agree well with the experimental values. The length and fineness of bamboo fiber can be calculated by the model.