A new type of low-stress cropping system based on hydraulic compensation is proposed in this paper. In order to solve the sudden change of critical loading force before and after the crack initiation at the tip of V-shaped notch and improve the quality of cropping section, it is proposed that the fracture process of the bar is divided into two stages. At the crack initiation stage, the calculation formula of the critical loading force is established by means of stress concentration factor and extended finite element method (XFEM). At the crack propagation stage, the calculation formula of the critical loading force is built by stress intensity factor and FRANC3D software. The concept of mutation coefficient Fp0.2/Fci is also put forward. The research results show that the influence of bottom corner radius s of V-shaped notch on Fp0.2/Fci is the largest; when s is between 0.12 mm and 0.3 mm, Fp0.2/Fci decreases sharply; when s is equal to 0.12 mm, there is no force mutation. The cropping results show that the high-quality cross-section is obtained by using the variable frequency-loading curve designed in this paper.