Abstract: Rock bending fractures are common in engineering activities in karst areas and can affect rock-mass integrity and engineering safety. To investigate the critical fracture behavior of rock masses and identify instability precursors, three-point bending tests were conducted on limestone specimens. Acoustic emission (AE) monitoring and digital image correlation (DIC) were used to develop a multi-parameter method for identifying precursors of critical failure in limestone under three-point bending. The results show that: (1) limestone bending failure exhibits clear brittle-fracture characteristics. Peak load and fracture energy first increase and then decrease with loading rate, and they vary rapidly within the range of 0.05-0.1 mm/min. (2) AE signals are most active during unstable crack propagation, when cumulative hits increase by more than 38% and reach a peak. After this peak, a main crack parallel to the loading direction forms. Cracks propagate mainly as tensile cracks during loading, accompanied by some shear cracks. (3) The shear-crack proportion, AE b-value, and critical-slowing-down index can be used as indicators for identifying precursors of critical bending failure in limestone. Among them, the shear-crack proportion shows a larger precursor response coefficient, an earlier response time, and higher reliability. This study provides an important reference for understanding the bending deformation and failure characteristics of limestone and for identifying disaster precursors in engineering rock masses.