Abstract: To improve the calculation accuracy of peridynamics and eliminate the limitations of the commonly-used Prototype Microelastic Brittle (PMB) peridynamic model, an extended micropolar peridynamic constitutive model has been developed, which can describe the internal length effect of non-local long-range forces and the mechanical properties of brittle materials such as rock and concrete. The corresponding peridynamic numerical system suitable for simulating multiple crack propagation in brittle materials has been proposed as well. The quantitative calculation accuracy of the proposed model has been verified, and the kernel function with the highest accuracy has been confirmed through the deformation analysis of a cantilever beam. The model and numerical methods are employed to simulate multiple crack growth in brittle plates subjected to uniaxial tension and compression, and the numerical results agree well with previous studies. The influence of initial angles, initial lengths of cracks on the failure mode and failure load of three-point bending beams has been investigated.