The load-crack mouth opening displacement (P-CMOD) curves and load-strain (P-ε) curves of autoclaved aerated concrete (AAC) specimens with different CF contents were obtained through three-point bending test. Then the cracking load ( \(P_{ini}\) ) and ultimate load ( \(P_{max}\) ) of the specimens were determined, and the corresponding initiation toughness ( \(K_{Ic}^{ini}\) ), instability toughness ( \(K_{Ic}^{un}\) ), and fracture energy ( \(G_{f}\) ) were calculated. In addition, the microstructure of carbon fiber reinforced AAC (CF/AAC) specimens was observed through scanning electron microscopy (SEM) test. The experimental results showed that the CF additives change the crack path of AAC, and the curvature of the crack path on the fracture surface increased with the increase of CF content. When the CF content increases from 0% to 0.5%, the \(P_{ini}\) , \(P_{max}\) , \(K_{Ic}^{ini}\) , \(K_{Ic}^{un}\) , and \(G_{f}\) of CF/AAC all show an increasing trend. When the CF content is 0.5%, the \(P_{ini}\) , \(P_{max}\) , \(K_{Ic}^{ini}\) , \(K_{Ic}^{un}\) , and \(G_{f}\) increase 87.47%, 128.52%, 87.5%, 182.48%, and 189.31%, respectively. The SEM results indicate that CF forms a disordered distribution in AAC matrix and adheres well to the AAC matrix, which effectively constrains the development of cracks.