The impact of the fiber reinforcement index （I_R） of basalt fiber bars （BFB） on the toughness index T_2（n-1）（n） of basalt fiber bar polymer concrete （BFB-PC） was investigated. The relationship between T_2（n-1）（n） and crack morphology was discussed by fractal theory. A calculation model of flexural toughness was established. The results indicate that， compared to PC without BFB， when I_R is 90.0， BFB has the best toughening effect on BFB-PC， and the flexural toughness of BFB-PC is increased by 8.39 times. The toughening thresholds of BFB are I_R=37.5 and I_R=90.0. When the I_R is the same， T_2（n-1）（n） of BFB-PC with the polymer-cement ratio of 0.06 is higher than that of concrete without polymer. The crack morphology becomes more complicated with the increases of I_R， and T_2（n-1）（n） has a quadratic function relationship with the fractal dimension of the crack. The model accurately expresses the toughening effect of BFB on PC beams， and the relative error between the experimental and theoretical values is less than 15.00%.