Method and blade monitoring system for monitoring bending moment of a wind turbine blade. The method comprises obtaining a first sensor set signal indicative of a first bending moment at a first sensor position different from the tip end along the longitudinal axis of the wind turbine blade, and estimating a bending moment at a first estimation position along the longitudinal axis based on the first sensor set signal, wherein the first sensor position is different from the first estimation position along the longitudinal axis. The blade monitoring system comprises a processing unit and an interface connected to the processing unit, the processing unit being configured for performing the method.