The Chinese code GB 50010—2010 Code for Design of Concrete Structures gives a formula to calculate the axial compression capacity of confined concrete columns by using the yield strength of spiral and welded ring stirrups, and it assumes that the stirrups have yielded under the peak compressive stress of confined concrete. In fact, the stirrups can yield only when the volume stirrup ratio exceeds a certain limit. Meanwhile, the code has not codified the case for square concrete columns confined by grid stirrups. To explore the tensile stress level in grid stirrups under peak loading, we conducted axial compression tests on 42 high-strength concrete square columns confined by grid stirrups. The axial compressive strength of the unconfined concrete ranged between 50.0 MPa and 68.0 MPa, and the stirrups were made of HRB400, HRB500, HRB600, PC800, PC1 270, and steel wire with standard tensile strength of 1 570 MPa. Results show that majority of the stirrups did not yield under the peak compressive stress of the confined concrete. The axial compressive strength of concrete, volume stirrup ratio, and hoop spacing had great impact on the stress level of stirrups. On the basis of the test data of unyielding stirrups under peak compressive load, a formula for calculating the actual tensile strain of stirrups under peak compressive load was established, and a method of calculating the lower limit value of volume stirrup ratio for stirrups capable of yielding under peak compressive load was proposed. By considering the effect of the actual tensile stress of stirrups on peak compressive stress and strain of the confined concrete, the full curve equation for the compressive stress-strain relationship of grid stirrups confined high-strength concrete was established, which provided reference for the engineering application and calculation of square concrete columns confined by grid stirrups.