Herein, the first-order vortex beam was generated from Yb∶phosphate (Yb∶QX) oscillator possessing defect-spot mirror. Moreover, the relationships among the optical-to-optical conversion efficiency of the vortex beam, defect-spot diameter, and laser mode diameter and shape inside the cavity were minutely evaluated. Results indicate that in Z-shaped solid-state lasers, when the diameter of laser mode inside the cavity is approximately 4--6 times the defect-spot diameter, the ratio of the laser spot diameter in the tangent and sagittal planes (i.e., ellipticity) is approximately 80%, and the pump power is 1.59 W, the first-order vortex beam with the highest optical-to-optical conversion efficiency of 7.7% is obtained. This study can provide a theoretical and experimental basis for generating first-order vortex beam with high conversion efficiency in Z-shaped solid-state lasers using defect-spot.