The study of the mechanical properties of engineering materials such as cement paste and cement mortar has become increasingly important with the needs of engineering applications. In this paper, the three- point bending beam test for the U-shaped notched beam specimens with three types of strengths, groove depths and groove root radii were carried out with a new fracture criterion_the theory of critical distance, which consist of 27 combinations in total. Theoretical parameters were obtained and verified by formula calculation and finite element simulation, and a new formula for calculating the maximum load of the notched beam was derived. The observation shows that for the maximum load, the calculated value of the new formula has good precision. The strength of mortar increases, while the theoretical parameter L value increases.