In order to study the bonding performance of the threaded glass fiber reinforced polymer(GFRP) bar and the concrete interface, three groups of pullout specimens were prepared from several GFRP bars of different diameters. A central pullout test was conducted on these specimens after 28d standard curing. Experimental results show that the bonding strength between GFRP bar and concrete increases as the bar diameter increases. The failure modes and bonding stress slip curves of GFRP bars of different diameters change accordingly. For the GFRP bars with small diameters of 8mm and 12mm, the tested specimens are mainly pulled out and damaged, but the bonding stress slip curves in the two cases also differ greatly after the stage of elastic rise. The test specimens of the diameter of 8mm tend to fluctuate back and forth, and gradually flatten. While the bonding stress of the pullout specimen with a bar of diameter of 12mm continues to rise and then decreases, showing a hyperbolic mode. When the diameter of GFRP bars becomes larger(16mm), the splitting damage appeares in the specimens tested. After the bonding stress slip curve rises to the highest point, the bonding stress decreases rapidly. Finally, based on the theoretical model of the thick walled cylinder of elastic mechanics, the angle of the bonding failure surface of the threaded GFRP bars of different diameters was discussed and the formula for calculating the bonding strength was proposed. The comparative analysis with the relevant test results verifies that the calculation formula has excellent prediction accuracy for specimens with pullout failure.