Bamboo fiber （BF） was modified by alkali pretreatment and grafting of cellulose nanofibers （CNFs）. The modification mechanism of BF and the effect of different BF modification method on the mechanical properties of bamboo fiber reinforced cement matrix composite （BFRC） were analyzed by mechanical properties and microstructure tests. The results show that alkali pretreatment and grafting modification of CNFs can improve the compressive strength of BFRC to a certain extent. When the grafting content of CNFs is 40 g/m²， the flexural strength and impact strength of BFRC are higher than that of single alkali pretreatment. Alkali pretreatment removes most of the hemicellulose， pectin and part of the lignin and wax on the BF surface and increases its surface roughness， while grafting modification of CNFs modifies the original defects on the BF surface and reduces the porosity of the cement matrix.