Eucalyptus wood fiber was prepared to investigate the effects of alkali treatment on mechanism of extrusion bonding of the board. The results showed that hemi-cellulose and lignin were partly degraded during eucalyptus wood fiber were treated with alkali，these degraded substances could be copolymerized during hot-pressing，the influence of alkali (NaOH) on eucalyptus wood fiber was greater than the weak base (Na2CO3) and the processing time for 10 h was more appropriate，and the increased hydroxyl after defibrinating partly formed hydrogen-bonding during hot-pressing，resulting in strengthened fiber self-bonding.