The plasticity and strength of bamboo were required in processing bamboo fiber by machine. The aim of this work is to explore changes of mechanical properties lengthways tensile properties of bamboo after alkali treatment and focuses on a relation between lengthways tensile properties of alkali treated bamboo and the moisture content. Experiment shows that: above fibre saturation point, the lengthways tensile strength of bamboo increased with decrease of the moisture content with NaOH 2.5% soaking for 24h and bamboo plastic is good where the moisture content is between 40% ~ 90%. Mechanism of plastic deformation of bamboo was initially revealed in this paper. Thus, moisture content of bamboo was should controlled in 40 ~ 90% in processing thick bamboo fiber.